tests; update README

tests/main-enterprise-mutual-exclusivity-both.test.js

@@ -0,0 +1,16 @@
+import { DEFAULT_ENV } from "./main.js";
+
+// Verify `main` exits with an error when `enterprise` is used with both `owner` and `repositories` inputs.
+try {
+  // Set up environment with enterprise, owner, and repositories all set
+  for (const [key, value] of Object.entries(DEFAULT_ENV)) {
+    process.env[key] = value;
+  }
+  process.env.INPUT_ENTERPRISE = "test-enterprise";
+  process.env.INPUT_OWNER = "test-owner";
+  process.env.INPUT_REPOSITORIES = "repo1,repo2";
+  
+  await import("../main.js");
+} catch (error) {
+  console.error(error.message);
+}