add ESM-safe os test helpers and tsconfig for tests

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

__tests__/.helpers/os.ts

@@ -0,0 +1,30 @@
+/**
+ * Copyright 2025 actions-toolkit authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {jest} from '@jest/globals';
+import os from 'os';
+
+export const mockPlatform = (platform: NodeJS.Platform) => {
+  return jest.spyOn(os, 'platform').mockImplementation(() => platform);
+};
+
+export const mockArch = (arch: string) => {
+  return jest.spyOn(os, 'arch').mockImplementation(() => arch);
+};
+
+export const mockHomedir = (dir: string) => {
+  return jest.spyOn(os, 'homedir').mockImplementation(() => dir);
+};