Check only one class in JS decompiler tests to reuse test data

JVM tests check only single class result, in order to reuse test data
JS tests should do the same. CHECK_PACKAGE is introduced to override
this behaviour explicitly.

idea/testData/decompiler/decompiledTextJs/TypeAliases/TypeAliases.kt

@@ -26,3 +26,5 @@ class TypeAliases {
     @Suppress("TOPLEVEL_TYPEALIASES_ONLY")
     private typealias Parametrized<E, F> = Map<E, F>
 }
+
+// CHECK_PACKAGE