This ticket aims to clean-up the test helpers in analyze_plan.js

At the creation of this ticket there are four helpers getting the `winningPlan` from the explain output of a query.

specifically,

getWinningPlan: gets the winning plan from the queryPlanner sub-object of an explain output.

getWinningSBEPlan: gets the winning plan from the queryPlanner sub-object of an explain output of an SBE query plan.

getWinningPlanFromExplain: gets the winning plan from the explain output of an SBE query plan.

getWinningSBEPlanFromExplain: gets the winning plan from the explain output of an SBE query plan.

This ticket aims to de-duplicate these implementation and converge to using only one.

The goal is to use one general implementation which will subsume all others, and update all uses if these functions throughout the tests.