Configure strict testing via phpunit.xml.

Problem with removing it is that we would also likely want to remove it from `.gitignore` which makes generating the report less fun because it leaves an untracked directory around.  I like it more where it is now as we can just run `phpunit` directly (`./vendor/bin/phpunit`) with no args if we don't want to do coverage.

Also, `build.php` needs a coverage report in order to run (the code coverage report is only generated if some code coverage report is specified).  So removing coverage-html could only be done by instead adding something else.