test-lib: introduce API for verifying file mtime

test-lib: introduce API for verifying file mtime

Add functions `test_set_magic_mtime` and `test_is_magic_mtime` which can
be used to (re)set the mtime of a file to a predefined ("magic")
timestamp, then perform some operations and finally check for mtime
changes of the file.

The core implementation follows the suggestion from the
mailing list [1].

[1] https://lore.kernel.org/git/xmqqczl5hpaq.fsf@gitster.g/T/#u

Signed-off-by: Marc Strapetz <marc.strapetz@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>