Unit testing and MvxIoCSupportingTest
## 🏗 Enhancement Proposal

Recently I've been tasked with adding Tests to a project with MVVMCross, I came across a situation when I needed to assert the SUT had logged. It wasn't as easy as I'd hoped as the tests also need to output the logs at the test runner window, so this was implemented using MvxIoCSupportingTest.SetupTestLogger

This posed a problem as there is no suitable interface to mock so I was left to mock the concrete class. Ideally I would just verify that Log had been called but this method is not marked virtual so MOQ can't wrap it.
 
### Pitch

Tidy up MvxIoCSupportingTest and either simply mark TestLogger.Log as virtual and or add a ITestLogger interface.

This isn't a show stopper as workaround can be used by subclassing TestLogger and just adding a Virtual Log method there and calling the base.
