Fix flake: wait for codeinsights-db.sh to startup in CI

Created by: mrnugget

More info in ticket: #25285 (closed)

This does two things:

1. Do not use pgx in dbtesting.TimescaleDB, but use dbconn instead, because only that one respects the DB_STARTUP_TIMEOUT set in go-test.sh, which is executed in CI.
2. Switch from t.Fail, which marks the test as failed but continues execution, to t.FailNow, which marks the test as failed and stops execution. That fixes the nil-pointer panic in case the database connection couldn't be established.