Rewrite complement.sh (#9685)

This PR rewrites the original complement.sh script with a number of improvements:

* We can now use a local checkout of Complement (configurable with `COMPLEMENT_DIR`), though the default behaviour still downloads the master branch.
* You can now specify a regex of test names to run, or just run all tests.
* We now use the Synapse test blacklist tag (so all tests will pass).