insights: increase test db max_connections to 20 for parity with old image
Created by: coury-clark
Related to https://github.com/sourcegraph/sourcegraph/issues/32861#issuecomment-1077918386
After starting up the old Timescale image, it seems the default configuration was 20 max connections. So this sets the same for parity. We also had one test that wasn't appropriately cleaning up resources, so fixing that.
Test plan
Ran locally and CI to ensure test DB can connect