[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.

libraries/tools/kotlin-stdlib-gen/src/generators/unicode/GenerateUnicodeData.kt

@@ -147,7 +147,7 @@ fun main(args: Array<String>) {
                 StringLowercaseGenerator(wasmGeneratedDir.resolve("_StringLowercase.kt"), unicodeDataLines, KotlinTarget.WASM)
             )
 
-            val nativeTestDir = baseDir.resolve("kotlin-native/backend.native/tests/stdlib_external/text")
+            val nativeTestDir = baseDir.resolve("kotlin-native/runtime/test/text")
             stringCasingTestGenerator = StringCasingTestGenerator(nativeTestDir)
 
             // For debugging. To see the file content