How to reproduce:
1. Open the attached "New Unity Project (2).zip" project
2. Open the Test Runner window
3. Press Run all in player
4. Observe the player

Expected result: "All test(s) succeeded" message is shown in the player
Actual result: "ArgumentException: Type TestRunCallbackAttrbiute does not implement ITestRunCallback" error message is thrown in the player

Reproducible with: 1.1.14 (2019.4.3f1, 2020.1.0b15, 2020.2.0a17)
Could not test with: 2018.4.24f1 (TestRunCallback attribute not available)

Notes:
- Running tests with "Scripting Backend" property set to Mono and "Managed Stripping Level" property set to Disabled or Low results in all tests passing
- Running tests with "Scripting Backend" property set to IL2CPP and "Managed Stripping Level" property set to Low results in all tests passing
- Running tests with "Scripting Backend" property set to Mono or IL2CPP and "Managed Stripping Level" set to Medium or High results in an error message "ArgumentException: Type TestRunCallbackAttrbiute does not implement ITestRunCallback" being thrown
- Running tests on Windows or Mac player produces the same results (If the build fails on Mac, update the Newtonsoft Json to 2.0.0 in the Package Manager)