Improve test coverage if possible

[idg10]

Currently, AzureManagedIdentityTokenSource has no test coverage. Work out whether it's possible to test that reasonably. (It might not be, due to dependencies on Azure.)

Also, look into testing for the service collection extensions.

If testing turns out not to be practical for these, bump the IMM score (and add a note explaining why we're getting the points despite the apparent missing tests).