Rework the legacy tests and move them into the new structure

[oliverklee]

• ├── Comment
• │   └── CommentTest.php [TASK] Migrate part of CommentTests to unit tests #821
• ├── CSSList
• │   ├── AtRuleBlockListTest.php [TASK] Move some AtRulBlockList tests to unit tests #927
• │   ├── DocumentTest.php [TASK] Move DocumentTest to unit tests #931
• │   └── KeyFrameTest.php [TASK] Move KeyFrameTest to unit tests #822
• ├── OutputFormatTest.php
• ├── ParserTest.php
• ├── RuleSet
• │   ├── DeclarationBlockTest.php
• │   └── LenientParsingTest.php
• ├── SettingsTest.php [TASK] Move SettingsTest to the unit tests #810
• └── Value
•  | ├── CalcRuleValueListTest.php [TASK] Move CalcRuleValueListTest to unit tests #987
• | ├── SizeTest.php [TASK] Move SizeTest to unit tests #836
• | └── ValueTest.php [TASK] Move ValueTest to unit tests #837