The psr/log package used to have not only the Interface for PSR-3 Logger, but also actual implementations of the interface like the TestLogger.
The TestLogger could be used as mock for any PSR-3 Logger in your test cases.
However from v3 the TestLogger was removed, so that the psr/log package would focus solely on the Interface.
If you used the TestLogger in your project and you or some of your dependencies upgraded psr/log to >= v3 you most likely saw this error:
Class "Psr\Log\Test\TestLogger" not found