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

compiler/tests/org/jetbrains/jet/codegen/CodegenTestFiles.java

@@ -82,10 +82,6 @@ public class CodegenTestFiles {
         return psiFiles;
     }
 
-    public boolean isScript() {
-        return psiFiles.size() == 1 && psiFiles.get(0).isScript();
-    }
-
     public static CodegenTestFiles create(Project project, String[] names) {
         ArrayList<JetFile> files = new ArrayList<JetFile>();
         for (String name : names) {