gh-130197: Improve test coverage of msgfmt.py part 2

[tomasr8]

This is a followup to #133048

This PR adds more tests to msgfmt.py:

• general syntax error tests (mainly unclosed strings and invalid plural syntax)
• semantic errors such incorrect ordering of msgid/msgstr etc.., invalid plural indices and duplicate entries.

I verified that these are rejected by GNU's msgfmt.

Note that some tests are commented out. That is because msgfmt.py is currently accepting these as valid.
In a followup PR, I am planning to fix its parser and then we'll be able to uncomment all of theses cases.

• Issue: pygettext: Improve test coverage #130197

[tomasr8]

@serhiy-storchaka Do you think you could have another look? Thank you!