Refactor campaigns go tests

Created by: eseliger

This does some refactoring of test helpers, reduces test run time by 3s(!) in the enterprise main package, because we forgot to use our fast sleep in test that we implemented, but apparently never set the flag 🤦, and re-uses the globalDB in resolver tests, which reduces the overall test run time even further while maintaining the go like test workflow of having one top level test function per tested unit. Some benchmarks below. The memoized setup DB works well and I wouldn't know how it might cause problems - except for that -count=N with N > 1 is not working properly currently. I don't know if we ever use that or it's needed to work well, but if so I guess we could get around it with inserting repo names with ascending counts in the names or timestamps. [...], since those are essentially the only ones that cause trouble AFAICT.

In total, this gets rid of a couple redundant test helpers and reduces test runtime for the main package by roughly 3s, resolvers 4-5s and store around 1s.

Pro-tip: Best reviewed commit-by-commit. Side-note: I did one strange approach of memoizing the call to setupglobaltestdb before, that disappears in the last commit.

Before

ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns        9.939s 9.675s 9.979s 10.521s 9.909s 10.072s
ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns/resolvers      9.920s 10.040s 9.923s 11.047s 10.182s 10.391s
ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns/search 0.418s 0.265s 0.227s 0.230s 0.248s 0.248s
ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns/store  4.840s 4.845s 4.791s 4.649s 4.443s 4.795s

After

ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns        6.504s 6.153s 6.260s 6.370s 6.407s 6.372s
ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns/resolvers      5.118s 4.932s 5.011s 5.127s 5.093s 5.063s
ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns/search 0.225s 0.281s 0.218s 0.224s 0.228s 0.229s
ok      github.com/sourcegraph/sourcegraph/enterprise/internal/campaigns/store  3.645s 3.452s 3.358s 3.379s 3.570s 3.469s

Edit: I reverted the global db sharing for now, since it breaks -count=2 runs and I don't want to spend too much time on this right now.