kotlin.test: 'fail' returns Nothing, 'assertFails' returns Throwable (not nullable).

Js tests now fail at the first failed assertion.
#KT-10289 Fixed
#KT-10369 Fixed

libraries/kotlin.test/junit/src/main/kotlin/JUnitSupport.kt

@@ -35,7 +35,9 @@ object JUnitAsserter : Asserter {
         Assert.assertNull(message ?: "actual value is not null", actual)
     }
 
-    override fun fail(message : String?) {
+    override fun fail(message : String?): Nothing {
         Assert.fail(message)
+        // should not get here
+        throw AssertionError(message)
     }
 }