Testkit: Testprobe name

This PR makes it possible to specify names when creating probes inside tests:
``` C#
[Fact]
public void MyTest()
{
  var probe = CreateTestProbe("MyTestProbe");
  ...
}
```

## Note
This also includes the commit in PR #460 (which fixes bug #459) so merge that one first. I submitted them at the same time to speed up reviewing.