multi-target unit tests

[bitfaster]

This PR:

1. Enables unit tests to build and run on .NET 4.8, .NET Core 3.1 and .NET 6 targets.
2. Runs all targets on windows
3. Runs .NET6 on mac
4. Collects code coverage for 3.1 and 6.0, since .NET Framework is not well supported in dotnet test.

[bitfaster]

Looks like coverage result files will have target fwk like this:

dotnet test --no-restore --verbosity normal --no-build -f net6.0 /p:CollectCoverage=true /p:CoverletOutput=TestResults/ /p:CoverletOutputFormat=lcov --logger "trx;LogFileName=results6.trx"

Generates

This can be done per target, e.g.:

dotnet test --no-restore --verbosity normal --no-build -f netcoreapp3.1 /p:CollectCoverage=true /p:CoverletOutput=TestResults/ /p:CoverletOutputFormat=lcov --logger "trx;LogFileName=results3.trx"

dotnet test --no-restore --verbosity normal --no-build -f net48 /p:CollectCoverage=true /p:CoverletOutput=TestResults/ /p:CoverletOutputFormat=lcov --logger "trx;LogFileName=results4.trx"

However, 4.8 target fails to resolve the net standard BitFaster.Caching .dll so no tests run:

This appears to be caused by code coverage, because this command works correctly:

dotnet test --no-restore --verbosity normal --no-build -f net48 --logger "trx;LogFileName=results4.trx"

[coveralls]

Coverage: 96.437% (-0.2%) from 96.671% when pulling debcf53 on users/alexpeck/multitest2 into 8b62b7b on main.