In many places in test suite we have "sleep"s that do not have to be
there.

 - I do not simply see the point of the one in t3500.  It may be making
   sure that the timestamp order of commits generated during the test is
   stable, in which case test_tick is the right ingredient to use without
   wasting tester's time.

 - The one in t4011 is to make sure that the plumbing diff-index notices
   the stat-dirtyness of a removed then identically recreated symlink.
   Keeping the old symlink around to make sure that a newly created
   symlink gets different ino would be sufficient for that purpose.

 - The one in t7600 is to make sure that "git merge" does not get confused
   by stat-dirty "file" in the working tree.  Again, keeping the old file
   around and creating an identical copy to ensure a different ino would
   be sufficient for that purpose.

The "racy git" tests in t0010 are inherently about mtime between the index
itself and index entries.  The "sleep" in that test must stay as they are.

Signed-off-by: Junio C Hamano <gitster@pobox.com>