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

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

@@ -0,0 +1,20 @@
+package foo
+
+open class A(var a: Int) {
+
+    open fun Int.modify(): Int {
+        return this * 3;
+    }
+
+    fun eval() = a.modify();
+}
+
+class B(a: Int) : A(a) {
+    override fun Int.modify(): Int {
+        return this - 2;
+    }
+}
+
+fun box(): String {
+    return if ((A(4).eval() == 12) && (A(2).eval() == 6) && (B(3).eval() == 1)) "OK" else "fail"
+}