Remove codegen tests out of JetNpeTest
Sort out blackBox*() methods

- get rid of blackBox(), extract tests into files
- delete single-file usages of blackBoxMultiFile()
- inline some other blackBox methods
- suppress exception in blackBoxWithJava()
Refactor CodegenTestCase

- prohibit main(Array<String>) in favor of box(): String
- move all script-related code to ScriptGenTest
- remove unused environment-creating methods
- inline trivial methods & other minor stuff
Codegen tests: don't generate twice when a test fails
Extract classPathInTheSameClassLoader into a separate test

- class loader is now always created as if classPathInTheSameClassLoader
  = false, which was the case of every codegen test except kt2781, which
  is extracted into a separate file
- testNoClassObjectForJavaClass() is moved from StdlibTest since it has
  a custom createClassLoader() which doesn't work well in a java-interop
  case
- get rid of generateClassesInFileGetState
- several similar methods are inlined
Extract static functions to CodegenTestUtil

Add Nullable/NotNull annotations
NamespaceGenTest doesn't require stdlib
Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated
Remove generated codegen tests, move all testData to box/

A single test file will be generated out of box/ directory
Fix failing tests, copy testData

Some tests were using files that others called blackBoxFile() on. Since
those files are now moved to box/, copy them back to make the tests pass
Generate black box codegen tests
Extract some codegen tests to black box testData
Remove codegen tests with Java, move testData to boxWithJava/

There'll be a single generated test class like
BlackBoxCodegenTestGenerated
Remove generated black box java codegen test

Move all testData to boxWithJava/
Generate black box with java codegen tests