Refactor helper function to run/not run block of common code on specific platform

Use it in StringBuilder tests

libraries/stdlib/jvm/test/testUtilsJVM.kt

@@ -12,8 +12,7 @@ public actual fun assertTypeEquals(expected: Any?, actual: Any?) {
     assertEquals(expected?.javaClass, actual?.javaClass)
 }
 
-public actual fun testOnJvm(action: () -> Unit) = action()
-public actual fun testOnJs(action: () -> Unit) {}
+public actual val TestPlatform.Companion.current: TestPlatform get() = TestPlatform.Jvm
 
 @Suppress("HasPlatformType", "UNCHECKED_CAST")
 public fun <T> platformNull() = Collections.singletonList(null as T).first()