JS: create new common directory for all generated tests, migrate several tests there

js/js.translator/testData/box/extensionFunction/extensionUsedInsideClass.kt

@@ -0,0 +1,15 @@
+package foo
+
+class A(var a: Int) {
+    fun eval() = f();
+}
+
+fun A.f(): Int {
+    a = 3
+    return 10
+}
+
+fun box(): String {
+    val a = A(4)
+    return if ((a.eval() == 10) && (a.a == 3)) "OK" else "fail"
+}