[CI] Test on .NET 5.0

[ChrisMaddock]

Fixes #830 and also fixes #511

This runs the tests on .NET 5.0, where available. I thought this was important to at least test on .NET 5.0, as it's a platform I suspect a large number of our users will shortly be running on. I haven't yet seen any reason to add a .NET 5.0 build though - so this instead runs tests for the .NET Core 3.1 Engine/Console on .NET 5.0. I also didn't see a need to add a requirement for users to have .NET 5.0 installed to contribute, so if unavailable, the default is to provide a warning - assuming the tests will be run on all platforms in CI instead.

It also improves the CI's handling of .NET Core platforms, as per #511. The new behaviour:

• The Cake script will now locate the most appropriate runtime for the request - that being the highest major/minor version installed for the runtime requested. e.g. on my machine, the 3.1 tests will chose to run on 3.1.5 over 3.1.2.
• If a runtime isn't available, e.g. .NET 5.0:
  • When building locally through the Cake scripts, if a certain runtime isn't available the script will provide a warning.
  • When building in CI and a certain runtime isn't available, the script will error. This is with various known exceptions built in, such as the .NET Core 1.1 test running on Azure.

One day I hope the Engine itself will do all this boilerplate stuff for us...we'll get there! 😄

[mikkelbu]

LGTM. I've got a couple of questions/comments.

[ChrisMaddock]

Thanks Mikkel! I'll come back and fix up the problems at the weekend. 😄