Add tests

[frontsideair]

• [x] Add a single smoke test (branch change)
• [ ] Integrate testing with GitHub Actions
• [ ] Make test run on dev version
• [ ] Add a test for each supported package manager
• [ ] Add pull test (merge)
• [ ] Add pull test (rebase)
• [ ] Add pull test (fast-forward)
• [ ] Add reset/restore test
• [ ] Add checkout test (should not fail) (#39)
• [ ] Add single file checkout test
• [ ] Add switch test
• [ ] Add merge/rebase test
• [ ] Add stash test
• [ ] Add cherry-pick test
• [ ] Add revert test
• [ ] Add branch switch from yarnhook-installed to yarnhook-not-installed (should break, consult to husky) (#13)

[frontsideair]

Starting this now, finally!