Add tests for support of covariant return overrides

[vitek-karas]

In .NET 5 CoreCLR is adding support for covariant return overrides. See the language spec and the runtime design doc for more details.

The runtime support for CoreCLR is still WIP here.

There's also a user story tracking this work - 1096979.

[vitek-karas]

#1209 is tracking the investigation work of this, to determine the impact.

[vitek-karas]

It seems that linker should already support the feature fully. So the only missing bits are testing. That can be done only once both runtime and language support this fully (and once we move linker to those versions so that it can use the feature in tests).