Move folder exclusion from test execution to matrix generation

[janpio]

Currently we exclude some folder in the repo from running tests: https://github.com/prisma/prisma-examples/blob/cb99c8c51c710a5b500229b6de385eb81befddd3/.github/scripts/test-all.sh#L18

That leads to problems and confusion like this: https://github.com/prisma/prisma-examples/pull/3020#pullrequestreview-705687840 Looks like we are testing something, but we are really not.

So instead of skipping just running the tests, we could/should skip scheduling them: https://github.com/prisma/prisma-examples/blob/cb99c8c51c710a5b500229b6de385eb81befddd3/.github/workflows/test.yaml#L17-L38

The logic behind that lives in this file: https://github.com/prisma/prisma-examples/blob/cb99c8c51c710a5b500229b6de385eb81befddd3/.github/scripts/get-packages.js

And it seems as it might be as simple as removing the databases folder from the allow list.

One possible problem: There is no automated way that makes sure there are passing tests for the databases subfolders.