OmeroCpp: Export test library symbols

This is a change to fix the current windows build error which is a failure to link omero-test.

Note: Requires #2966 and #2990 to build cleanly.

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Testing: http://ci.openmicroscopy.org/job/OMERO-5.1-merge-cpp-win/ should go green.  You can also build by hand; just follow the steps in this job.