JS backend: test for KT-2901: nullable type in string template
Minor: add diagnostic test for entries in enum with abstract members
Do not report ABSTRACT_MEMBER_NOT_IMPLEMENTED on enum classes

Partial fix of KT-3454
Do not report NON_FINAL_MEMBER_IN_FINAL_CLASS on open members in enum classes
"abstract" modifier on enum class is an error

 #KT-3454 Fixed
Test code generation for enum deriving from a trait