Collect code coverage of the client in unit tests

[pavog]

This PR adds a coverage report to the unit tests.

tests.yaml

• PHP is set up with pcov.
• The unit tests write a clover report to coverage.xml. There is no coverage threshold, so the report does not fail the build.

phpunit.xml.dist

• Coverage is only collected for src/Client. The generated code in src/Api and src/Model is not tested directly.
• The file is reformatted with 4 spaces, and the unit suite is listed first.