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

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

@@ -0,0 +1,15 @@
+package foo
+
+class A(var a: Int) {
+
+    fun Int.modify(): Int {
+        return this * 3;
+    }
+
+    fun eval() = a.modify();
+}
+
+fun box(): String {
+    val a = A(4)
+    return if (a.eval() == 12) "OK" else "fail"
+}