build: Clean up test runner and make phpcs strict

* Move phpunit.xml to root (where it is auto-detected and no longer
  requires us to pass it by argument to phpunit).
* Rename phpunit.xml to phpunit.xml.dist so that users can have
  a local phpunit.xml.
* Make phpcs strict.
* Simplify up phpunit directory structure.
* Remove tests/run bash script. Use composer-test instead.