Bug 19775: Try & prevent Search/History.t to fail randomly

Bug 19775: Try & prevent Search/History.t to fail randomly

 #   Failed test 'There are still 8 searches after calling delete with an interval = 6 days'
 #   at t/db_dependent/Search/History.t line 185.
 #          got: '6'
 #     expected: '8'

 #   Failed test 'There are still 2 searches after calling delete with an interval = 2 days'
 #   at t/db_dependent/Search/History.t line 191.
 #          got: '0'
 #     expected: '2'
 # Looks like you failed 2 tests of 27.
[15:04:12] t/db_dependent/Search/History.t

It looks like it does not longer fail with this change (date comparaison wrongly rounded to second in mysql?)

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>