Provide option to exclude specific tests

[mthalman]

With the work to enable the execution of the scenario tests as part of the VMR build (dotnet/source-build#3819), it seems likely there will be times when we need to temporarily disable a specific test that is failing to unblock the build. This change would ideally be done in the test execution logic rather than directly modifying this repo to disable the test. So this would require a new command line option that would allow the caller to specify one or more test names that should be excluded from running.