Installing NUnit for C# builds

I'm wondering whether NUnit should be installed as default for C# builds to support unit testing. Having NUnit pre-installed would also allow the use of the new docker based build infra.

The core nunit  libraries already come with the `mono-complete` package, however the `nunit-console` is missing.

I know it is possible to install NUnit with NuGet, however invoking it from bash like `mono Mpdeimos.Playground.Templating/packages/NUnit.Runners.2.6.3/tools/nunit-console.exe $(find . -path "*/test-src/bin/*_Test.dll")` is not straight forward and the nunit runner is usually no project dependency.

/cc @joshua-anderson @akoeplinger @nterry.