Speed up xsite tests

The cross-site tests are much slower than the regular core tests: it takes more time to run the 233 xsite tests than the almost 6k regular tests.

There are several reasons for this:

1. XSite tests cannot run in parallel because they all use the same bridge cluster configuration (will be handled in ISPN-5476)
2. Shutting down the cache managers after a test takes a long time because of JGRP-1927
3. They use AbstractInfinispanTest.eventually() with higher timeouts, which also makes them wait more between condition checks.