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/main-token-get-owner-unset-repo-unset.test.js

@@ -0,0 +1,25 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when neither the `owner` nor `repositories` input is set.
+await test((mockPool) => {
+  delete process.env.INPUT_OWNER;
+  delete process.env.INPUT_REPOSITORIES;
+
+  // Mock installation id request
+  const mockInstallationId = "123456";
+  mockPool
+    .intercept({
+      path: `/repos/${process.env.GITHUB_REPOSITORY}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(
+      200,
+      { id: mockInstallationId },
+      { headers: { "content-type": "application/json" } }
+    );
+});