Fix order-dependent test failures

What does this MR do and why?

Prevent extra DB connection pools from leaking to succeeding tests and also improves our connection check initializer so that it works even with multiple connection pools for the same DB name.

Failure can be reproduced with:

bin/rspec "spec/tasks/gitlab/backup_rake_spec.rb" "spec/lib/initializer_connections_spec.rb"

Note that backup_rake_spec.rb deletes the db/ directory so you need to checkout db/ from master again if you want to re-run the specs. Already created a separate issue for this: #462975 (closed)

Related to #444963 (closed) #444962 (closed) #444961 (closed) #444960 (closed)

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