fix: run coverage before pytest

[12rambau]

Fix #65

According to https://stackoverflow.com/questions/62221654/how-to-get-coverage-reporting-when-testing-a-pytest-plugin the plugins themselves cannot be tested with pytest-cov as the coverage session needs to start before the plugins are actually loaded.

Following the instructions of the answer I changed the noxfile file content to launch coverage manually and export the report in xml.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7c6649c) 0.00% compared to head (309bae1) 96.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##            main      #66       +/-   ##
==========================================
+ Coverage   0.00%   96.00%   +96.00%     
==========================================
  Files          2        2               
  Lines         75       75               
==========================================
+ Hits           0       72       +72     
+ Misses        75        3       -72     

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