Fix pyuvsim test CI for new package structure

[bhazelton]

Description

We recently restructured pyuvsim to use the src layout. This fixes the way pytest is called in light of the new structure.

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation change (documentation changes only)
• [ ] Version change
• [x] Build or continuous integration change

Checklist:

• [x] I have read the contribution guide.
• [x] My code follows the code style of this project.

Build or continuous integration change checklist:

• [ ] If required or optional dependencies have changed (including version numbers), I have updated the readme to reflect this.
• [ ] If this is a new CI setup, I have added the associated badge to the readme and to references/make_index.py (if appropriate).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.92%. Comparing base (7d3804f) to head (dd87ace).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1426   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          37       37           
  Lines       20868    20868           
=======================================
  Hits        20852    20852           
  Misses         16       16           

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