Simplify tests on not-null assertions

No need to call blackBoxFile() on a useless test data file. Also creating
environment logic is simplified
Delete StdlibTest and StdlibTestForever

Move remaining tests to AnnotationGenTest (where they did belong to in the
first place)
Move multi-file codegen testData to boxMultiFile/

There'll be an auto-generated test over this directory
Autogenerate codegen multi-file tests

Based on boxMultiFile/ directory
Switch class loading logic in blackBoxWithJava tests

BoxWithJava tests now by default are loaded with the classloader which has
test's classpath in itself, as in the former ClassPathInTheSameClassLoaderTest
Move blackBox() methods to AbstractBlackBoxCodegenTest