Hopefully fix flaky gitserver tests by not using global test DB
Created by: mrnugget
Example failure: https://buildkite.com/sourcegraph/sourcegraph/builds/112624#0eb45a1a-6584-4f27-978d-17232578adac
Created by: mrnugget
Example failure: https://buildkite.com/sourcegraph/sourcegraph/builds/112624#0eb45a1a-6584-4f27-978d-17232578adac