Remove dependency on the deprecated DbUnit

[blackcoder87]

Description

The goal is to be able to run unit tests under PHP 8 and newer.

• DbUnit is abandoned and is currently used to do unit tests against the database.
• phpunit/phpunit-mock-objects as separate part is abandoned and got merged into PHPUnit (version 7?) in early 2018.
• Currently a really old version of PHPUnit is used. An upgrade to a newer version is hard as Ilch for example supports PHP 7.0 and 7.1. Maybe remove support of PHP 7.0, 7.1 and maybe even 7.2 earlier than ~June 2022 if necessary.

See:
#475
#475 (comment)
https://phpunit.de/supported-versions.html
#482

• Removed dependency on the deprecated DbUnit by replacing it with an own solution (not as feature rich, but works for current tests).
• Adjusted existing unit tests to work with the new replacement.
• Fixed various issues found by the IDE.

There are a few more points that can be targeted, but I leave that for later pull requests. For example:

• Support loading of other file types if needed.
• Accept directly an array and therefore avoid having to access files (currently yml).
• Cache content of files to not read them too often?

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

[c0r1an]

@blackcoder87 vielen Dank, hast du toll gemacht :)