Add Playwright browser tests

[elsassph]

Fix browser tests

Some Mocha browser tests were present in the repository - however they wouldn't work in their current state:

• HTML page referenced incorrect Lightning runtime,
• Some Flexbox tests would not pass; it may be that Lightning engine diverged a little bit from HTML flexbox which was used as reference. I added a workaround to ignore divergences.

Added Playwright

Added an automation script to run those browser tests.

This required to adjust the Mocha tests because:

• All the Mocha tests were running in a way hard to track with Playwright,
• The tests create many Lightning stages which never stop running, which would be very heavy for CI; now the stages are stopped after the tests end.

Added Playwright CI check

Playwright tests now run as part of CI checks.