[ticket/12996] Fix reliability issue in flock test.

$delta was always an int - so, this test would sometimes fail if you
happened to call time() /very/ close to a 1s boundary.

Found by HHVM's continuous testing.

PHPBB3-12996
Merge pull request #2892 from bantu/ticket/12996

[ticket/12996] Fix reliability issue in flock test.

* bantu/ticket/12996:
  [ticket/12996] Fix reliability issue in flock test.