obsolescence: add test case B-7 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 B-7: Prune above non-targeted common changeset

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-B7.t

@@ -0,0 +1,100 b''
+============================================
+Testing obsolescence markers push: Cases B.7
+============================================
+
+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 7: Prune on non-targeted common changeset
+
+B.7 Prune above non-targeted common changeset
+=============================================
+
+.. (very similar to B1, but the prune changeset is unknown on remote)
+..
+.. {{{
+..     ⊗ B
+..     |
+..     ◕ A
+..     |
+..     ● O
+.. }}}
+..
+.. Marker exist from:
+..
+..  * B (prune)
+..
+.. Command runs:
+..
+..  * hg push -r O
+..
+.. Expected exclude:
+..
+..  * B (prune)
+
+Setup
+-----
+
+  $ . $TESTDIR/testlib/exchange-obsmarker-util.sh
+
+Initial
+
+  $ setuprepos B.7
+  creating test repo for test case B.7
+  - pulldest
+  - main
+  - pushdest
+  cd into `main` and proceed with env setup
+  $ cd main
+  $ mkcommit A
+  $ hg push -q ../pushdest
+  $ hg push -q ../pulldest
+  $ mkcommit B
+  $ hg prune -qd '0 0' .
+  $ hg log -G --hidden
+  x  f6fbb35d8ac9 (draft): B
+  |
+  @  f5bc6836db60 (draft): A
+  |
+  o  a9bdc8b26820 (public): O
+  
+  $ inspect_obsmarkers
+  obsstore content
+  ================
+  f6fbb35d8ac958bbe70035e4c789c18471cdc0af 0 {f5bc6836db60e308a17ba08bf050154ba9c4fad7} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  $ cd ..
+  $ cd ..
+
+Actual Test
+-------------------------------------
+
+  $ dotest B.7 O
+  ## Running testcase B.7
+  # testing echange of "O" (a9bdc8b26820)
+  ## initial state
+  # obstore: main
+  f6fbb35d8ac958bbe70035e4c789c18471cdc0af 0 {f5bc6836db60e308a17ba08bf050154ba9c4fad7} (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
+  ## post push state
+  # obstore: main
+  f6fbb35d8ac958bbe70035e4c789c18471cdc0af 0 {f5bc6836db60e308a17ba08bf050154ba9c4fad7} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pushdest
+  # obstore: pulldest
+  ## pulling "a9bdc8b26820" from main into pulldest
+  pulling from main
+  no changes found
+  ## post pull state
+  # obstore: main
+  f6fbb35d8ac958bbe70035e4c789c18471cdc0af 0 {f5bc6836db60e308a17ba08bf050154ba9c4fad7} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
+  # obstore: pushdest
+  # obstore: pulldest