Do not load detached plugins during tests (#2829)

* Do not load detached plugins during tests.
Also warn about plugins which force us to create classes.jar.

* Porting POM fix from #2795: we must allow snapshots from repo.jenkins-ci.org so as to be able to test upstream component PRs.

* Fixed failures in tests which assumed detached plugins were being loaded.

* jenkins-test-harness 2.20

* Unreproducible test failure on CI, probably due to timing.