tests: ensure a status will have non ambiguous mtime in some race test

For the test to work, we need some mtime to be recorded. For them to be recorded, they need to be "clearly in the past", otherwise edit with the same mtime would be possible.

Strictly speaking there might be file system with a minimal mtime increment
longer than one second. However it is unlikely that we will run the test on
them for now. We can be smarter about this in the future if it becomes
necessary.

Differential Revision: https://phab.mercurial-scm.org/D11779

tests/test-dirstate-race.t

@@ -89,6 +89,7 b' anyway.'
   > rm b && rm -r dir1 && rm d && mkdir d && rm e && mkdir e
   > EOF
 
+  $ sleep 1 # ensure non-ambiguous mtime
   $ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py
   M d
   M e