tests: remove a sub-test since we are done with it in test-run-tests.t

removing test-failure.t to make sure that in next patch when
we re-run failed tests using --retest, it won't get considered
unnecessarily.

Differential Revision: https://phab.mercurial-scm.org/D8963

tests/test-run-tests.t

@@ -844,6 +844,8 b' Accept the fix'
     $ echo 'saved backup bundle to $TESTTMP/foo.hg'
     saved backup bundle to $TESTTMP/*.hg (glob)<
 
+  $ rm test-failure.t
+
 Race condition - test file was modified when test is running
 
   $ TESTRACEDIR=`pwd`