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'
Minor: remove warnings
Introduce utility method for working with multifile tests
Fix invalidating built-ins psi in consecutive test runs
Add structure view source roots
Tests for file structure
Revise JetStructureViewElement

- remove 'my' prefix
- inline ItemPresentation
- use descriptor renderer instead of custom one
Set up rendering for structure elements
Enable members sorting in file structure view
Show members from supertypes in file structure view (KT-4448)

 #KT-4448 Fixed
Add location string showing where method or property is defined
Better caching: do not recount when field true value is null
Fix empty name for class initializer in project view when "Show Members" enabled