Run slow tests with a large database on CI

[danielballan]

In #588 we added a test that places 10k rows in the nodes table, in order to effectively exercise an index.

We followed a common pytest pattern for slow tests: adding slow marker to the test, and adding -m 'not slow' to our pytest invocation.

To run this test---and perhaps future ones like it---on CI, we should:

• Move the creation of that large database into a dedicated fixture that has a stable database name, rather than uuid4, and uses an existing database if it exists
• On CI, mount an external volume in the container for postgres data.
• Cache the volume between CI runs