The integration test that was broken was:
```
--- FAIL: TestCreateInvalidExperiment (0.35s)
experiment_integration_test.go:192: expected a nil builder here
```
While there improve the documentation of the ExperimentSession
and see there's a method that we are not using.
This diff is a cleanup that I come up with while working
on https://github.com/ooni/probe/issues/2184.