[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/utils.kt

@@ -0,0 +1,38 @@
+/*
+ * 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
+
+import kotlin.test.*
+
+public actual fun assertTypeEquals(expected: Any?, actual: Any?) {
+    if (expected != null && actual != null) {
+        assertTrue(expected::class.isInstance(actual) || actual::class.isInstance(expected),
+                "Expected: $expected,  Actual: $actual")
+    } else {
+        assertTrue(expected == null && actual == null)
+    }
+}
+
+public actual val TestPlatform.Companion.current: TestPlatform get() = TestPlatform.Native
+
+public actual val isFloat32RangeEnforced: Boolean get() = true
+
+public actual val supportsOctalLiteralInRegex: Boolean get() = true
+
+public actual val supportsEscapeAnyCharInRegex: Boolean get() = true
+
+public actual val regexSplitUnicodeCodePointHandling: Boolean get() = false
+
+public actual object BackReferenceHandling {
+    actual val captureLargestValidIndex: Boolean get() = true
+
+    actual val notYetDefinedGroup: HandlingOption = HandlingOption.THROW
+    actual val notYetDefinedNamedGroup: HandlingOption = HandlingOption.THROW
+    actual val enclosingGroup: HandlingOption = HandlingOption.MATCH_NOTHING
+    actual val nonExistentGroup: HandlingOption = HandlingOption.THROW
+    actual val nonExistentNamedGroup: HandlingOption = HandlingOption.THROW
+    actual val groupZero: HandlingOption = HandlingOption.THROW
+}