Launchable NUnit integration

See https://www.launchableinc.com/docs/features/predictive-test-selection/requesting-and-running-a-subset-of-tests/subsetting-with-the-launchable-cli/zero-input-subsetting/#dotnet-test--nunit for the user documentation

Development instruction

• dotnet build to build
• dotnet test to run unit tests. dotnet test -l:nunit to produce TestResults.xml

Release instruction

• Update the version number in Launchable.NUnit/*.csproj
• dotnet pack -c Release to produce a release mode package. See ls **/*.nupkg
• Upload this to https://www.nuget.org/packages/Launchable.NUnit with a browser. (TODO: automate this via https://learn.microsoft.com/en-us/nuget/quickstart/create-and-publish-a-package-using-the-dotnet-cli)