Fix PHPUnit config to restore Codecov support

[mbabker]

After #2818 it looks like Codecov hasn't been working because the coverage config was looking for the wrong source path. That's been fixed. A couple other minor QoL improvements:

• Updates .gitignore to ignore a phpunit.xml at root instead of inside the tests folder, making local edits easier (don't have to add the -c tests/phpunit.xml config anymore to not use the dist file)
• Adds the code coverage to .gitignore (generally won't be there, but doesn't hurt since I copied the command out of the GHA config to make sure it actually generated a coverage file)
• Updates the PHPUnit config to use the XSD from the local install instead of a URL, as this regularly falls out-of-date

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.84%. Comparing base (0632ab1) to head (63fa9bd).
Report is 58 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2842      +/-   ##
==========================================
+ Coverage   78.75%   78.84%   +0.09%     
==========================================
  Files         163      167       +4     
  Lines        8593     8638      +45     
==========================================
+ Hits         6767     6811      +44     
- Misses       1826     1827       +1     

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[phansys]

Thank you @mbabker!