apply different quality of life improvements to the shadow
css unit tests:
- refactor the tests so that they are nicely divided in multiple files
in a logical manner instead of having most of them all in a single big file
- remove the css normalization logic inconsistently used throughout the tests, which
causes tests to be inconsistent and it also introduced unintuitive checks
- provide a shared shim utility function (instead of re-defining it
multiple times)
- add a `toEqualCss` matcher that can be used in the tests in order to
match css strings without caring about spacing and indentation
PR Close#48443
