No tests of .NET Core 3.1 packages

[CharliePoole]

When the .NET Core 3.1 package was added, no checks were added to package-checks.cake. The package checks aren't real tests but they at least confirm that the right files are in each package, avoiding the need for manual verification.

[CharliePoole]

I believe this also applies to the .NET Core console package.

[CharliePoole]

@ChrisMaddock I'll go ahead and do this as it's simple. I put it in the 3.13 milestone. I assume we'll eventually decide whether to actually have a 3.13 or to just put these changes into 4.0.