Steps to reproduce:
1. Download the SingletonSystem package
2. Open the user's project
3. Open the “Package Manager” window (Window → Package Manager)
4. Click on a plus in the top left corner and select “Add package from disk”
5. Add the “SingletonSystem” package
6. Open the “Test Runner” window (Window → General)
Expected result: There is a “SingletonAttributeTestbed” test
Actual result: No tests to show
Reproducible with: 1.1.33 (2020.3.40f1, 2021.3.11f1, 2022.1.18f1, 2022.2.0b8, 2023.1.0a10)
Reproduced on: macOS Monterey 12.5.1 (Intel)
Workaround: Copy and paste the package into the package folder instead of adding it via Package Manager (works only with 2021.3.11f1)