[FLINK-6395] [tests] Mark test bases as abstract

[zentol]

No description provided.

[tillrohrmann]

Not sure whether marking classes without abstract methods abstract makes sense. If we want to prevent instantiations of these classes we could make the constructor also protected.

[zentol]

As you can see in the UnaryOperatorTestBase the constructors are already protected. The thing is that junit complains since the classes contain junit annotations (@After, @Parameterized, @RunWith) class but can't be instantiated. When i run these test in IntelliJ they are marked as failed tests due to an initialization error.

Making the classes abstract makes it clear they aren't actually self-contained test classes.

[tillrohrmann]

Alright, now I understand. +1 for merging.