Quality of life improvements for feature sets

While feature sets solve a real problem, they're currently quite unwieldy. All callsites which use the featureset helper have the same pattern of creating a feature set, looping around all feature sets, creating a new testcase, setting up the context and then finally disabling the given set of features in that context. This is extremely repetitive and easy to get wrong. As a result, they're more of a burden to use. Furthermore, they do have a serious bug which causes us to not generate all combinations of feature flags, but only a small subset of those.

This MR fixes both issues. First, we now generate all combinations of feature flags. Second, it introduces a new Run() function which handles all of the setup for us. E.g. the typical use previous to that refactoring

       featureSets, err := testhelper.NewFeatureSets([]featureflag.FeatureFlag{
                featureflag.GoListConflictFiles,
       })
       require.NoError(t, err)

       for _, featureSet := range featureSets {
               t.Run("disabled "+featureSet.String(), func(t *testing.T) {
                       ctx, cancel := testhelper.Context()
                       defer cancel()

                       ctx = featureSet.Disable(ctx)

                       testcase(t, ctx)
               })
       }

... now looks like this:

       testhelper.NewFeatureSets([]featureflag.FeatureFlag{
               featureflag.GoListConflictFiles,
       }).Run(t, func(t *testing.T, ctx context.Context) {
               testcase(t, ctx)
       })

I've also added some testcases to finally verify they're doing the correct thing.