22.0-unit_test-TrkTrackCollectionMerger

Introduces a very basic unit test for testing only the merging part of the TrackCollectionMerger, by introducing two collections of length=1 in storegate and then checking that the merged result has length 2. The TrackCollectionMerger code itself was modified to allow the merging part to execute without a TrackSummary update for the purposes of the test. Testing the TrackSummary update will be added later.