When writing a jstest that makes assertions about a plan, you usually want to use the helpers in analyze_plan.js so that your test will handle all the different plan shapes that can happen in passthrough suites.

However, there are a lot of helpers, which makes it hard to know which to use.

Also, some of the helpers apparently don't work in all cases. I was recently surprised that aggPlanHasStage() apparently does not check the mergerPart of the pipeline. (Other cases like $lookup not explaining its subpipeline, would require server changes to fix.)

Let's:

• review the list of helpers and decide which ones need to exist, vs which ones can be removed or combined,
• fix obvious problems like aggPlanHasStage() not checking the entire plan.