Connection tests use a lot of memory

[alexjpwalker]

Problem to Solve

When running the feature files connection/session.feature and connection/database.feature in parallel, our CI machines run out of memory.

Proposed Solution

We could do one of the following:

• Reduce the number of concurrent databases opened in these features (currently 6)
• Increase the amount of memory in the CI machine that runs them (currently 2GB)
• Limit the parallelisation of test runs using --jobs=1 (this is what we currently do, along with a TODO message saying we should fix this issue. It may be that this is, however, the best solution, in which case we should remove that TODO)