[JENKINS-22395] Taking advantage of BuildReference.clear (just introduced in...

[JENKINS-22395] Taking advantage of BuildReference.clear (just introduced in b6226ad2) to simplify test by not requiring a custom build reference holder just to simulate GC.
Confirmed that dropLinksAfterGC and dropLinksAfterGC2 both fail in the expected way (b1a.nextBuild == b2) after commenting out the call to createReference().clear() in dropLinks.
(Also that they fail as expected in assertNotSame if the reference is not cleared at all.)