Unify tests for unmodifiable emulation

[niloc132]

From @jnehlmeier in #9860

Maybe we should refactor a bit in a follow up commit that introduces a helper class to check immutability of collections/sets, lists, maps. We now have many tests that need it: CollectionsTest, ListTest, SetTest, MapTest, CollectorsTest. Each doing it a bit differently and not covering everything.

The CollectionsTest.doTestModificationsToList method could be made public and static and reused for other List cases, and easily generalized for Collection/Set - adding another validator for Map should be easy as well.

Note that there are no direct tests at this time for Collections.unmodifiableMap nor Collections.unmodifiableSet, but other implementations rely on their correctness.