Resolve that `stub_experiments(foo: :bar)` stubs all experiments.

I think there's just some more effort required here. If it's an unknown experiment, it should only be stubbed for experiments with the specified name etc.

The following discussion from !60 (merged) should be addressed:

• @jejacks0n started a discussion:

  It's maybe worth calling out here, that we actually don't have an experiment that's called NameExperiment, and so we actually stub Gitlab::Experiment in these cases.. I wonder if we want to add better behavior here to raise an exception or something?