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

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

@@ -0,0 +1,13 @@
+package foo
+
+fun Int.same(): Int {
+    return this
+}
+
+fun Int.quadruple(): Int {
+    return same() * 4;
+}
+
+fun box(): String {
+    return if ((3 + 4).quadruple() == 28) "OK" else "fail"
+}