Update OMERO agent string in ``common`` unit test

This PR fixes https://trac.openmicroscopy.org.uk/ome/ticket/11388. When connecting to QA to test the upgrade check class, the test was using ``OMERO.test`` as the user agent. That caused QA to redirect the request. After changing to ``OMERO.server`` (one of the valid user agents in QA's model), no HTML code is present in stdout.

To test:
verify that the output of running ``./build.py -f components/common/build.xml test`` doesn't have any HTML code in the output and that all tests pass.