Fix warnings in JVM codegen tests

Add generics where needed, add nullability annotations, etc.
Also delete some testcases from ArrayGenTest which will never be supported
Minor, improve test case for KT-1157

Multithreaded version was not working correctly and was throwing NPEs in the
log sometimes
Minor, change test case according to its name
Fix logic of forcing method return type to be wrapped

Return type must be wrapped if any super-method in the hierarchy, not just the
direct super-method, returns a reference type (as opposed to a primitive type)

Actually there was a test reflecting this, but it wasn't running because of an
unfixed diagnostic error
Check for syntax/diagnostic errors in some tests
Disallow extension properties with backing fields

 #KT-1682 Fixed
Don't add initializer to extension property on override/implement
Provide implicit receiver to extension property initializer

This is safe because initializers for extension properties are forbidden
anyway, we'll just won't be generating noisy error messages about non-existent
'this'