Bug 19385: Fix random t/Calendar.t failure - clear the cache before

Bug 19385: Fix random t/Calendar.t failure - clear the cache before

The cache 'exception_holidays' may be populated when we run these tests,
we need to clear it before the tests are run.

Test plan:
  prove t/db_dependent/Circulation/CalcDateDue.t  t/Calendar.t

Without this patch, t/Calendar will fail with:
  #   Failed test 'Exception holiday is not a closed day test'
  #   at t/Calendar.t line 159.
  #          got: '1'
  #     expected: '0'
  # Looks like you failed 1 test of 38.

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