Add tests

- [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)