Include coverage for all source files

[bosschaert]

Description

Previously coverage figures were only on source files touched by unit tests, which meant that if you added tests to a previously untested sourcefile, it would typically bring the overall coverage down, unless that new file was immediately close to 100% covered.

This change computes coverage figures on all source files, which is more truthful.

It will seriously bring the coverage figure down, but the new coverage figure is the correct one.

Also updated .nycrc.json to accept the current figure as the minimum one.

Types of changes

A build system fix. No change to the actual product code.

Checklist:

• [X] I have signed the Adobe Open Source CLA.
• [X] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [X] All new and existing tests passed.

[codecov[bot]]

Codecov Report

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

Project coverage is 47.08%. Comparing base (48867b1) to head (2cc1d06).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #58 +/- ## =========================================== - Coverage 82.72% 47.08% -35.64% =========================================== Files 14 35 +21 Lines 909 1597 +688 =========================================== Hits 752 752 - Misses 157 845 +688 ```

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