tests: add a short `sleep` in test-status.t

With dirstate-v2 and rhg both enabled, this test would sometimes fail for me
with:

```
--- tests/test-status.t
+++ tests/test-status.t#dirstate-v2.err
@@ -943,7 +943,7 @@
   $ rm subdir/unknown
   $ hg status
   $ hg debugdirstate --all --no-dates | grep '^ '
-      0         -1 set                 subdir
+      0         -1 unset               subdir
```

Meaning that `status` did not write a directory mtime in the dirstate
as expected. This can happen if the observed mtime of the directory is
the same as "current time" at the start of `status`. This current time
is obtained by creating a temporary file and checking its mtime.

Even with ext4 on my system being able to store nanosecond precision,
identical mtime for successive but separate operations is still possible
becuse the kernel may cache the current time:
https://stackoverflow.com/a/14393315/1162888

0.1 second should be enough for this cache to be updated, without
significantly slowing down the test.

Differential Revision: https://phab.mercurial-scm.org/D11900

tests/test-status.t

@@ -941,6 +941,7 b' It is still not set when there are unkno'
 Now the directory is eligible for caching, so its mtime is save in the dirstate
 
   $ rm subdir/unknown
+  $ sleep 0.1 # ensure the kernel’s internal clock for mtimes has ticked
   $ hg status
   $ hg debugdirstate --all --no-dates | grep '^ '
       0         -1 set                 subdir