Add GHA wich runs coredev robottests with built bundle

[petschki]

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The idea here:

1. checkout buildout.coredev into tests/
2. patch mxcheckouts.ini to always checkout plone.staticresources
3. compile bundle into tests/src/plone.staticresources with yarn build:webpack:tests
4. run tests with ROBOTSUITE_PREFIX=ONLYROBOT bin/test --all -t ONLYROBOT like jenkins does.

[thet]

@petschki LGTM, basically.

I wonder if this burns too much CI server resources - e.g. when you create a PR and then update it - the whole CI pipeline would run again.

I'm OK merging this, but I'd keep an eye on how it behaves in real.