dslx: make sure netemx tests generate correct observations

[bassosimone]

In ooni/probe-cli#1388, we introduced tests using netemx where we don't check for observations, but it would be more robust if we add extra checks to make sure the observations are correct.

The overall objective here is to cleanup and review dslx, then start using it in production and rewrite experiments using it. In turn, by doing that, we'll enable collecting better throttling metrics for experiments.