JS: update backend tests to respect new front-end rules for external declarations

js/js.translator/testData/box/expression/function/_common.kt

@@ -6,6 +6,6 @@ fun String.replaceAll(regexp: String, replacement: String): String = replace(Reg
 
 inline fun String.search(regexp: RegExp): Int = asDynamic().search(regexp)
 
-external class RegExp(regexp: String, flags: String = "") {
+external class RegExp(regexp: String, flags: String = noImpl) {
     fun exec(s: String): Array<String>? = noImpl
 }

js/js.translator/testData/box/expression/function/mangling.kt

@@ -6,11 +6,13 @@ public fun public_baz(i: Int) {
     log = "public_baz"
 }
 external public fun public_baz(a: String) {
+    noImpl
 }
 
 internal fun internal_baz(i: Int) {
 }
 internal external fun internal_baz(a: String) {
+    noImpl
 }
 
 private fun getCurrentPackage(): dynamic = js("_").foo
@@ -18,6 +20,7 @@ private fun getCurrentPackage(): dynamic = js("_").foo
 private fun private_baz(i: Int) {
 }
 private external fun private_baz(a: String) {
+    noImpl
 }
 
 public class PublicClass {