Branch tests: use less indent-y featureset idiom

Continue amending the code changed in e87b76da ("featureset: Simplify execution with a new Run() function", 2020-11-03) to use the *.Run() idiom to use the testWithFeature() function instead.

This is better because when we add/remove these features we won't have this indent churn. There's a trivial cost that e.g. TestFailedUserDeleteBranchDueToHooks() does some needless setup now, but the CPU wastage is trivial, and it avoids future bugs as stuff that's supposed to be tested with/without the feature in that function would be put above the feature marker.

The two [Tt]estSuccessfulGitHooksForUserCreateBranchRequest() functions had also drifted apart in some previous changes, fix that while we're at it.

This also makes things more consistent with new code I'm adding in !2814 (closed).