Add more tests to check mrow fallback when computing preferred logical widths (#47094)
- Refactor code for generating invalid MathML markup
Since the logic for generating invalid MathML markup will be also used by future tests for testing logical preferred widths, computed baseline, etc. move this into a separate file.
- Add more tests to check mrow fallback when computing preferred logical widths
We currently check that certain invalid MathML markup falls back to being laid out as an <mrow> element. Add further tests to check that the preferred logical widths also fallback to being computed in the same way as an <mrow> element.
- Add tests for computing first line baseline
As part of testing that the layout falls back to that of an mrow, also test that the first line baseline falls back to that of an mrow.