Use same routines to install Mathesar in testing as in production

[dmos62]

Both in production and in testing we have separate routines for installing Mathesar schemata. The fact that we use different routines in backend and in frontend is a problem, because that implies smaller test coverage. I ran into an annoying bug that only showed up in the CI, but not when testing locally, because of this: the bug was in the production Mathesar install routines and that was being called when setting up the CI container, but not when running tests locally. We want tests to be as similar to production as possible.

As of time of writing, the routine used in production is db.install.install_mathesar. Fixtures in charge of this in testing are in conftest.py.