Dry run mode so that tests can verify that the entry point hasn't been broken

Updating from `beta1.20574.7` to `beta2.21617.1` caused the app to stop working because I hadn't seen https://github.com/dotnet/command-line-api/issues/1537:

```
Unhandled exception: System.ArgumentException: Service not found for type System.Uri.
```

I'd like to be able to write a unit test that passes a set of arguments to the app entry point and verifies that no such issue is incurred, but without actually invoking the handler. I thought of a `[dryrun]` directive. Maybe even cooler would be if this was possible in the test code:

```cs
object?[] handlerArguments = Handler.Verify(() => Program.Main(new[] { "arg1", "arg2" }));
// Assert that the arguments are as expected
```