Make tests run independently for each package in the monorepository

[lukasholzer]

Currently all tests and formatting checks are executed from the repository root. To make a partial migration of typescript possible we need to split down tests per package. This is a prerequisite for #4472

• This means each package under packages/* should have a own test npm script inside the package.json.
• So that we can run all the tests with npm run test --workspaces or lerna maybe.
• Aditionally we can do the same for formatting and linting the codebase as well.
• Integrate the whole pipeline in the github actions.
• try to share the ava.config.js between the packages but have a config in each package.
• fix the tests to reflect different paths

Non Goals:

• modify the testrunner
• don't mess with the parallel execution in the CI (try to keep the PR simple)