Dev test ci

[valentina-s]

This PR set up working CI for testing. Changes:

• workflow -> workflows
• renamed test.py to test_plot.py since pytest expects underscore to discover the test files
• fixed the fixture call
• pinned pandas to an older version (later need to be switched to 2)
• removed coverage since it is not tested

@ldr426 look at it so you are aware of the changes but we can merge it right away since I tested it is working. From now on run pytest locally before submitting a PR.

[ldr426]

Thanks for the modification @valentina-s!