Bug 20191: Use Test::MockTime to hold time still

Bug 20191: Use Test::MockTime to hold time still

The problem is the calls to HTTP::OAI::Header, etc.
may reference gmtime which is continually changing by
the second. By forcing time to lock for all the tests,
except the last one, we can be assured things should
not fail.

TEST PLAN
---------
install libtest-mocktime-perl
apply the patch
restart_all
in kshell, prove t/db_dependent/OAI/Server.t
run koha qa test tools

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>