circle tests flakily failing with 'Pool is closed' exception thrown

[jrgm]

e.g., https://circleci.com/gh/mozilla/fxa-auth-db-mysql/515 trying to build v1.110.0 for a third time and failing

However, master has built and passed the tests.

Note: It's not always the same unit test that fails.

[jrgm]

And on the sixth try there was a build.