Improve pytest developer experience

[dmos62]

Problem

• Our pytest test suites take too long to run. The db/tests/ suite alone takes 15 minutes to run on my machine (my i5-4670K is dated, but not outdated). This degrades ability to quickly iterate.
• Also, our tests are brittle, in that whether they pass might depend on the order in which they are run. Them being order-dependent makes it hard to run tests out-of-order, which is often preferable (--ff option runs failed and unrun tests first).
• Further, our fixtures are bloated and difficult to understand. Because of this, a developer will spend more time writing his tests (or editing others') and the result will be worse.
• Futher, our test coverage is inefficient due to heavy overlap, meaning that many aspects of Mathesar are tested many times over. This also contributes to brittleness, since one change might affect many tests.

Proposed solution

• To reduce runtime: refactor fixtures to allow for running multiple tests in parallel;
• To address order-dependence: setup pytest to run tests in random order by default;
• To improve readability: establish a coherent fixture system;
• To improve overlap: identify most expensive tests and factor out overlapping coverage.

Additional context

Issue related to test coverage overlap: #1138

[kgodey]

@dmos62 Is this fixed by your refactor?

[dmos62]

@kgodey the #1222 refactor made runtime a non-issue, improved fixture coherence and improved test hermitization.

Problems still left to address:

• Fixture coherence/readability can be further improved;
• order dependence is vastly improved, but random order execution is still worth exploring to further improve robustness;
• test overlap (testing the same thing many times) is still severe;
• instantiating a lot of engines on the backend threatens to bottleneck how many parallel test runners we can use.

[kgodey]

There is a separate issue for connection management: #592

@dmos62 Can you open separate issues for each of the other three items so that we can work on them separately?

[dmos62]

Split this issue into four issues (see mentioned-in issues above).