Fix failing acquisition date search test

Due to changes in openmicroscopy/openmicroscopy#2847 making the image acquisition date optional one test using a fake file is now failing, see [1]. This PR fixes that test using openmicroscopy/bioformats#1241 It also breaks the search tests using dates into two to make for neater parametrization.

All modified tests should pass.

1. http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-integration-python/201/testReport/test.integration.clitest.test_search/TestSearch/test_search_dates_data2_/