test: integration tests for main.js (#56)

Part of https://github.com/actions/create-github-app-token/issues/43

This PR adds tests for
[`main.js`](https://github.com/actions/create-github-app-token/blob/main/lib/main.js),
similar to [the tests that already exist for
`post.js`](https://github.com/actions/create-github-app-token/tree/main/tests).

Specifically, it tests that:
- `main` exits with an error when `GITHUB_REPOSITORY` is missing.
- `main` exits with an error when `GITHUB_REPOSITORY_OWNER` is missing.
- `main` successfully obtains a token when…
- …the `owner` and `repositories` inputs are set (and the latter is a
single repo).
- …the `owner` and `repositories` inputs are set (and the latter is a
list of repos).
- …the `owner` input is set (to an org), but the `repositories` input
isn’t set.
- …the `owner` input is set (to a user), but the `repositories` input
isn’t set.
  - …the `owner` input is not set, but the `repositories` input is set.
  - …neither the `owner` nor `repositories` input is set.

❧

Architecturally, in order to keep individual tests concise, this PR adds
`tests/main.js`, which:
- sets commonly-used inputs, environment variables, and mocks, then
- calls a callback function that can edit the variables and add
additional mocks, then
- runs `main.js` itself.

The `tests/main-token-get-*.test.js` test files run `tests/main.js` with
various scenario-specific callback functions.

tests/index.js

@@ -7,7 +7,12 @@ const tests = readdirSync("tests").filter((file) => file.endsWith(".test.js"));
 
 for (const file of tests) {
   test(file, async (t) => {
-    const { stderr, stdout } = await execa("node", [`tests/${file}`]);
+    // Override Actions environment variables that change `core`’s behavior
+    const env = {
+      GITHUB_OUTPUT: undefined,
+      GITHUB_STATE: undefined,
+    };
+    const { stderr, stdout } = await execa("node", [`tests/${file}`], { env });
     t.snapshot(stderr, "stderr");
     t.snapshot(stdout, "stdout");
   });