* Get rid of all remaining JUnit 4 usages except in prober & proxy subprojects
Caveat: Test suites aren't yet implemented in JUnit 5 so we still use the ones
from JUnit 5 in the core subproject.
* Fix some build errors
This adds an entriesToImmutableMap() collector that can be used in place of
toImmutableMap(Map.Entry::getkey, Map.Entry::getValue()).
It also fixes up some existing calls that use toImmutableMap() when terser
alternatives exist.