[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/numbers/MathExceptionTest.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2018 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.
+ */
+
+package test.numbers
+
+import kotlin.math.*
+import kotlin.test.*
+
+class MathExceptionTest {
+
+    @Test fun exceptions() {
+        assertFails { Double.NaN.roundToLong() }
+        assertFails { Double.NaN.roundToInt() }
+
+        assertFails { Float.NaN.roundToLong() }
+        assertFails { Float.NaN.roundToInt() }
+    }
+
+}