Exit with non-zero exit code when exit was called from some test

This is a feature request. Currently, if some test calls exit, phpunit stops and ends with a zero exit code.

This is dangerous as phpunit will pass in CI even if the test is broken and later tests did not run at all.

I propose phpunit to register a shutdown function and when it is invoked before phpunit completes, exit with a non-zero code there. This way, the exit code will be properly non-zero.