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/snapshots/index.js.md

@@ -4,6 +4,110 @@ The actual snapshot is saved in `index.js.snap`.
 
 Generated by [AVA](https://avajs.dev).
 
+## main-missing-owner.test.js
+
+> stderr
+
+    'GITHUB_REPOSITORY_OWNER missing, must be set to \'<owner>\''
+
+> stdout
+
+    ''
+
+## main-missing-repository.test.js
+
+> stderr
+
+    'GITHUB_REPOSITORY missing, must be set to \'<owner>/<repo>\''
+
+> stdout
+
+    ''
+
+## main-token-get-owner-set-repo-set-to-many.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner and repositories set, creating token for repositories "actions/create-github-app-token,actions/toolkit" owned by "actions"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-set-repo-set-to-one.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner and repositories set, creating token for repositories "actions/create-github-app-token" owned by "actions"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-set-to-org-repo-unset.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `repositories not set, creating token for all repositories for given owner "actions"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-set-to-user-repo-unset.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `repositories not set, creating token for all repositories for given owner "smockle"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-unset-repo-set.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner not set, creating owner for given repositories "actions/create-github-app-token" in current owner ("actions")␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-unset-repo-unset.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner and repositories not set, creating token for the current repository ("create-github-app-token")␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
 ## post-token-set.test.js
 
 > stderr