Implement dotnet-watch tests

The scenario-tests should have a test(s) for dotnet-watch. This is something that has regressed several times in source-build scenarios in the past. It feels like a scenario that should be covered in the scenario-tests. Currently there is a [source-build smoke-test](https://github.com/dotnet/dotnet/blob/main/test/Microsoft.DotNet.SourceBuild.SmokeTests/DotNetWatchTests.cs) that covers this. The scenario tests should remove the need for the smoke-test.