Fix TestConcurrentTransactions test by closing transaction

Created by: mrnugget

The test failed for me on a separate branch due to a timeout: it panicked after 10m. See build: https://buildkite.com/sourcegraph/sourcegraph/builds/168105#0182b53e-7a71-471b-a45e-e9125de6ed1e

I'm 99% it's unrelated to my code, especially since it only shows up in the backcompat test run and my changes are backwards compatible and shouldn't have anything to do with transactions executing pg_sleep.

So, what this does:

• it closes the transaction at the end of the test (pretty sure that this lead to timeouts).
• it inserts something into the test table (so it does what it says in the description)
• uses this NotEmpty test helper cause I couldnt' resist

Test plan

• Ran this locally multiple tests
• CI