Support XDEBUG_MODE environment variable #834

Summary

PHPUnit only understands Xdebug modes set in php.ini when checking for coverage availability.

When running:

$ XDEBUG_MODE=coverage ./vendor/bin/phpunit --coverage-text

It is not recognised that the mode is set.

Current behavior

PHPUnit shows a warning:

Warning:       xdebug.mode=coverage has to be set in php.ini

How to reproduce

• Any repository with phpunit available in vendor/bin/phpunit.
• Run: XDEBUG_MODE=coverage php -n -dzend_extension=xdebug vendor/bin/phpunit --coverage-text

Expected behavior

Code coverage should have been run, as it would have been with:

php -n -dzend_extension=xdebug -dxdebug.mode=coverage ./vendor/bin/phpunit --coverage-text