Make it easier to run a single playwright test

[mtlynch]

Related: https://github.com/tiny-pilot/tinypilot/issues/1694

We originally designed the run-e2e-tests script so that the caller could pass Playwright command-line flags, and the script would pass them through to Playwright, so you could do this:

./dev-scripts/run-e2e-tests --grep 'shows privacy policy'

And then the script would launch Playwright with:

npx playwright test --grep 'shows privacy policy'

We lost this functionality as part of https://github.com/tiny-pilot/tinypilot-pro/pull/851

Now that we're writing more e2e tests, it would be handy if the run-e2e-tests script made it easier for us to pick out specific tests to run from our suite.