[FIR JS] Add more tests for JS_NAME_CLASH and JS_FAKE_NAME_CLASH

^KT-59425 Related
^KT-59370 Related

compiler/testData/diagnostics/testsWithJsStdLib/name/classInheritanceExtensions.fir.kt

@@ -0,0 +1,11 @@
+// FIR_DIFFERENCE
+// This case is only relevant for the JS Legacy BE and is not applicable to the JS IR backend,
+// as the IR BE can resolve such name collisions.
+
+open class Class {
+    fun Int.test() {}
+    val Int.test
+        get() = 0
+}
+
+class MyClass1 : Class()