Add rule checking test and implementation class are in same package

[mslowiak]

This will add the library rule GeneralCodingRules.testClassesShouldResideInTheSamePackageAsImplementation(..) which will test that implementation and test class reside in the same package. The rule can e.g. detect mismatches like com.myapp.correct.SomeClass and com.myapp.wrong.SomeClassTest.

Resolves: #475

[codecholeric]

Thanks a lot for your contribution!! 😄 Looks very good in general 👍 I just made some small adjustments, mainly

• adjust the message a little to be more consistent with other messages
• use common testing infrastructure to test the rule

I just adjusted it, since I want to release these days, so I cut down the round trip a little. I hope that's okay with you! 🙏