test(robot-server): Check the number of returned commands in persistence snapshot tests

[SyntaxColoring]

Overview

robot-server's persistence snapshot tests currently only check that the server can return some number of run commands. They don't check that the server returns the correct number, or the correct contents. That was good enough before, but it's dicey now because we're doing a bunch of database migration stuff in this release that deals with commands. (e.g. #14348, #14465.)

This PR asserts that the server returns the correct number of commands. Because of time constraints, we're still not attempting to check that the contents are correct—that's RSS-213.

Test Plan

I've run these tests on a commit before all of v7.2.0's database changes, to be more certain that we're locking in good numbers instead of bad numbers.

Changelog

• For each run in each snapshot, hard-code the expected number of commands.
• Fix a funny test bug where it was fetching commands redundantly, because the code to do that was accidentally nested inside an unrelated loop.

Review requests

None in particular.

Risk assessment

Very low.

[sfoster1]

Yeah, that looks like a good idea, thanks!