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

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

@@ -0,0 +1,7 @@
+package foo
+
+fun <T> T.toPrefixedString(prefix: String = "", suffix: String = "") = prefix + toString() + suffix
+
+fun box(): String {
+    return if (("mama".toPrefixedString(suffix = "321", prefix = "papa") == "papamama321")) "OK" else "fail"
+}