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

js/js.translator/testData/box/callableReference/function/stringNativeExtension.kt

@@ -0,0 +1,8 @@
+package foo
+
+fun box(): String {
+    var s = "abc"
+    assertEquals("ABC", (String::toUpperCase)(s))
+
+    return "OK"
+}