Remove final keyword from all test class declarations

Discussion and consensus: https://github.com/exercism/xjava/issues/373

Marking our test classes as final is (1) more syntax for a learner to parse, and (2) not increasing the clarity of the writer’s intent given the context of an individual problem’s codebase. We should remove it from all test class declarations.

[stkent]

@morrme thanks for the latest round of removals! 😍

[FridaTveit]

Is there some artificial problem structure we could come up with to ‘introduce’ the concept of public? Perhaps exercises that provide helper files (enums, etc.) could place them in a support package within the main package, thereby forcing the use of the public modifier?

@stkent, I think this sounds like a great idea! 😃