Add tests fixes

- Added filter to phpunit config
- This enables support for coverage support
- Moved unit tests to dedicated directory
- Also created constants for the locations

Add tests fixes

- Added filter to phpunit config
- This enables support for coverage support
- Moved unit tests to dedicated directory
- Also created constants for the locations of the fixtures and mocks for
testing so we do not have to use relative paths in tests
- Updated phpunit to 8
- Added type information to tests
- Added return type and parameter type declarations for tests
- Added strict types to tests
- CS fixes in tests
- Aligning arrays, consistent string concatenation, removed unused local
vars
- Added constant for test cache directory
- Make the autoloader also properly work on windows
- Windows does not care about the directory separator used so just trim
both variant from the end.

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