Phpunit: Add minimum coverage option to fail if coverage falls below a threshold

[jdufresne]

When looking at coverage tools for other languages they frequently provide a configuration option to fail if coverage falls below a threshold. In coverage.py, the option is --fail-under. I think this feature would be useful in PHPUnit as well.

Some additional option to phpunit.xml such that coverage below a threshold results in some kind of failure that returns a non-zero exit status.

This feature would help continuous integration systems that want to catch commits that significantly drop coverage.

[redaxmedia]

+1

[sebastianbergmann]

I do not thinks that functionality like this belongs in PHPUnit.