[13.0][IMP] project_purchase_link: Add tests for purchase invoice buttons

[skeller1]

This PR is adding tests for purchase invoice and purchase invoice lines button call

https://codecov.io/gh/OCA/project/src/13.0/project_purchase_link/models/project_project.py

[github-actions[bot]]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days. If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[pedrobaeza]

/ocabot merge patch

[OCA-git-bot]

On my way to merge this fine PR! Prepared branch 13.0-ocabot-merge-pr-744-by-pedrobaeza-bump-patch, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at 055ee9f70a2b406ff4730d8e2a4703a50388809f. Thanks a lot for contributing to OCA. ❤️