Allow a directory of OME-XML files to be specified for testEqualOMEXML

This is intended to be used like this:

ant -Dtestng.omexmlDirectory=/path/to/samples -Dtestng.directory=/path/to/data test-automated

or:

ant -Dtestng.omexmlDirectory=/path/to/samples/leica/ -Dtestng.directory=/path/to/data/leica/ test-automated

where /path/to/samples/ is a clone of samples.git.

Fixes ticket #9403.