tests: clarify that duplicate flag processors is not an error

The flag processors test for duplicate processors for a single flag
was misleading because the file from the previous test case caused it
to fail (making the "echo 'this should fail' > file" part
irrelevant). Let's remove the leftover from the previous test case to
make it clear that duplicate flag processors results only in a
warning.

Note that duplicate flag processors would have resulted in a failure
(not just a warning) until ea1c2eb7abd3 (extensions: catch uisetup and
extsetup failures and don't let them break hg, 2017-06-06). I remember
expressing my concern about ending up with half-loaded extensions. It
would be pretty unfortunate to have double-encoded revlog content
enter a repo, so maybe we should reconsider?

Differential Revision: https://phab.mercurial-scm.org/D201

tests/test-flagprocessor.t

@@ -152,6 +152,8 b''
   $ hg commit -Aqm 'fail+base64+gzip+noop'
   abort: missing processor for flag '0x1'!
   [255]
+  $ hg forget fail-base64-gzip-noop
+  $ rm fail-base64-gzip-noop
 
 # TEST: ensure we cannot register several flag processors on the same flag
   $ cat >> .hg/hgrc << EOF
@@ -162,8 +164,6 b''
   $ echo 'this should fail' > file
   $ hg commit -Aqm 'add file'
   *** failed to set up extension duplicate: cannot register multiple processors on flag '0x8'.
-  abort: missing processor for flag '0x1'!
-  [255]
 
   $ cd ..