refs https://github.com/TryGhost/Toolbox/issues/207
- we want to start writing some E2E tests that involve automated
pointing and clicking around the frontend of Ghost to test that
members of Ghost sites can still do what we expect
- we've decided to look in to Playwright for this - it looks __really__
nice
- this is a VERY basic first test - it'll check for a 200 on the
homepage of whatever we provide as the TEST_URL env variable, or
default to a (manually-run) Ghost instance on port 2368
- also adds a `yarn test:browser` command to run the tests using the
Playwright CLI, and a sample GitHub Action workflow which we can
manually run with a site URL
- there's a lot more to add here in terms of test framework but this
gets us started
