Run tests on PHPUnit 9 and PHP 8

[SimonFrings]

PHPUnit 9.3 released a new schema for the phpunit.xml configuration file. I had to migrate the file to the new format in order to avoid the warning. PHPUnit Versions older than 9.3 have to use the phpunit.xml.legacy configuration file, because the new format is unknown for them.

For further details concerning this pull request look into graphp/graphviz#46.
This pull request builds on top of #132.

It's also possible to run this code with PHP 8 🎉

$ docker run -it --rm -v `pwd`:/data --net=host --workdir=/data php:8.0.0 php vendor/bin/phpunit
PHPUnit 9.5.8 by Sebastian Bergmann and contributors.

...............................................................  63 / 201 ( 31%)
............................................................... 126 / 201 ( 62%)
....................................I.......................... 189 / 201 ( 94%)
............                                                    201 / 201 (100%)

Time: 00:00.480, Memory: 42.43 MB

OK, but incomplete, skipped, or risky tests!
Tests: 201, Assertions: 527, Incomplete: 1.

[clue]

@SimonFrings Thanks for the update, changes LGTM!