new tests for registerJobs method

This commit adds tests to verify registering of gearman jobs.
Most of the single label scenarios are tested, however due to
weirdness with the HudsonTestCase class the multiple label
scenarios don't work the same as in normal Jenkins operation so
I could not create automated test for it.

Change-Id: I2e2ac5c51efd13d560c575310da4b8f8fb58f05b