Bug 18794: Fix test ListMetadataFormats in OAI/Server.t

Bug 18794: Fix test ListMetadataFormats in OAI/Server.t

This test may fail on slow servers, it compares the response date with
'now', but both can differ a bit.

https://jenkins.koha-community.org/job/Koha_Master_D8/198/consoleFull

   Failed test 'ListMetadataFormats'
   at t/db_dependent/OAI/Server.t line 150.
     Structures begin differing at:
          $got->{responseDate} = '2017-06-12T14:31:51Z'
     $expected->{responseDate} = '2017-06-12T14:31:50Z'

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