obsolescence: add test case B-6 for obsolescence markers exchange

About 3 years ago, in August 2014, the logic to select what markers to select on
push was ported from the evolve extension to Mercurial core. However, for some
unclear reasons, the tests for that logic were not ported alongside.
I realised it a couple of weeks ago while working on another push related issue.
I've made a clean up pass on the tests and they are now ready to integrate the
core test suite. This series of changesets do not change any logic. I just adds
test for logic that has been around for about 10 versions of Mercurial.

They are a patch for each test case. It makes it easier to review and postpone
one with documentation issues without rejecting the wholes series.

This patch introduce case B6: Pruned changeset with precursors not in pushed set

Each test case comes it in own test file. It help parallelism and does not
introduce a significant overhead from having a single unified giant test file.
Here are timing  to support this claim.

# Multiple test files version:
# run-tests.py --local -j 1 test-exchange-*.t
53.40s user 6.82s system 85% cpu 1:10.76 total
52.79s user 6.97s system 85% cpu 1:09.97 total
52.94s user 6.82s system 85% cpu 1:09.69 total

# Single test file version:
# run-tests.py --local -j 1 test-exchange-obsmarkers.t
52.97s user 6.85s system 85% cpu 1:10.10 total
52.64s user 6.79s system 85% cpu 1:09.63 total
53.70s user 7.00s system 85% cpu 1:11.17 total

tests/test-exchange-obsmarkers-case-B6.t

@@ -0,0 +1,117 b''
+============================================
+Testing obsolescence markers push: Cases B.6
+============================================
+
+Mercurial pushes obsolescences markers relevant to the "pushed-set", the set of
+all changesets that requested to be "in sync" after the push (even if they are
+already on both side).
+
+This test belongs to a series of tests checking such set is properly computed
+and applied. This does not tests "obsmarkers" discovery capabilities.
+
+Category B: pruning case
+TestCase 6: Pruned changeset with precursors not in pushed set
+
+B.6 Pruned changeset with precursors not in pushed set
+======================================================
+
+.. {{{
+..   B ø⇠⊗ B'
+..     | |
+..   A ○ |
+..     |/
+..     ● O
+.. }}}
+..
+.. Marker exist from:
+..
+..  * `B ø⇠⊗ B'`
+..  * B' prune
+..
+.. Command run:
+..
+..  * hg push -r O
+..
+.. Expected exchange:
+..
+..  * `B ø⇠⊗ B'`
+..  * B' prune
+
+Setup
+-----
+
+  $ . $TESTDIR/testlib/exchange-obsmarker-util.sh
+
+Initial
+
+  $ setuprepos B.6
+  creating test repo for test case B.6
+  - pulldest
+  - main
+  - pushdest
+  cd into `main` and proceed with env setup
+  $ cd main
+  $ mkcommit A
+  $ mkcommit B0
+  $ hg up --quiet 0
+  $ mkcommit B1
+  created new head
+  $ hg debugobsolete `getid 'desc(B0)'` `getid 'desc(B1)'`
+  $ hg prune -qd '0 0' .
+  $ hg log -G --hidden
+  x  f6298a8ac3a4 (draft): B1
+  |
+  | x  962ecf6b1afc (draft): B0
+  | |
+  | o  f5bc6836db60 (draft): A
+  |/
+  @  a9bdc8b26820 (public): O
+  
+  $ inspect_obsmarkers
+  obsstore content
+  ================
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  $ cd ..
+  $ cd ..
+
+Actual Test
+-------------------------------------
+
+  $ dotest B.6 O
+  ## Running testcase B.6
+  # testing echange of "O" (a9bdc8b26820)
+  ## initial state
+  # obstore: main
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pushdest
+  # obstore: pulldest
+  ## pushing "O" from main to pushdest
+  pushing to pushdest
+  searching for changes
+  no changes found
+  remote: 2 new obsolescence markers
+  ## post push state
+  # obstore: main
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pushdest
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pulldest
+  ## pulling "a9bdc8b26820" from main into pulldest
+  pulling from main
+  no changes found
+  2 new obsolescence markers
+  ## post pull state
+  # obstore: main
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pushdest
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pulldest
+  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+