3.x: remove `test` from `testXXX` method names, create a validator.

[akarnokd]

A lot of test methods were named like testXXX in the 0.x-1.x era and we didn't want to change those for 2.x. Now is the time to get rid of the redundant naming from all test methods.

In addition, a validator should be added that checks all test Java files and reports if it finds the pattern void\s+test[a-zA-Z0-9] in any file. See this validator for an example of finding and checking source code files.

[akarnokd]

Closing via #6525 & #6528.