[K/N] Migrate all stdlib_external tests to new testing infra ^KT-61259

- Move backend.native/tests/stdlib_external to runtime/test This mirrors
  tests location (in relation to sources) on other backends.
- Remove all backend.native/tests tasks using stdlib_external.
  Additionally remove now unused KonanGTest.
- Include complete native-wasm/test and runtime/test in
  native/native.test
- In runtime/test/numbers/MathTest.kt leave only native-specific part.

kotlin-native/runtime/test/text/StringEncodingTestNative.kt

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+@file:OptIn(FreezingIsDeprecated::class)
+
+package test.text
+
+
+internal actual val surrogateCodePointDecoding: String = "\uFFFD".repeat(3)
+
+internal actual val surrogateCharEncoding: ByteArray = byteArrayOf(0xEF.toByte(), 0xBF.toByte(), 0xBD.toByte())