* upgrade phpunit, test with low and high deps
* work around possibly-anonymous test cases introduced by newer PHPUnit
* Alternative TestCase::getName() nullability workaround
Previous workaround was failing due to PHP warnings on 7.1 or 7.2
(depending on specific signature). There's just no signature that would
be working for all 4 variants of (ver / dep) matrix.
* don't disable xdebug if it's not enabled
* allowed 7.0/high to fail until PHPUnit 6.5.10 is released
see sebastianbergmann/phpunit#3209