CDRIVER-4341 unskip /Stepdown/not_primary_keep

[kevinAlbs]

Summary

• unskip /Stepdown/not_primary_keep
• separate /Stepdown tests into /pooled and /single

Background & Motivation

/Stepdown/not_primary_keep was unskipped and run in a patch build with all variants and tasks with names matching ".*replica-noauth.*: https://spruce.mongodb.com/version/651c0c8957e85a9882f5109e. Restarting the tasks five times resulted in success (excluding a System Failure downloading 7.0 server).

I am not sure what caused the reported failure and if anything has changed to resolve. To help diagnose possible future failures, the /Stepdown tests are split into separate /pooled and /single tests. At present, both a pooled client and single-threaded client are tested in the scope of one test:

Begin /Stepdown/not_primary_keep, seed 1761489099
    { "status": "pass", "test_file": "/Stepdown/not_primary_keep", "seed": "1761489099", "start": 331.882479, "end": 333.485490, "elapsed": 1.603011  }

With this PR, the test is split into:

Begin /Stepdown/not_primary_keep/single, seed 959976514
    { "status": "pass", "test_file": "/Stepdown/not_primary_keep/single", "seed": "959976514", "start": 1205.916470, "end": 1205.998609, "elapsed": 0.082139  },
Begin /Stepdown/not_primary_keep/pooled, seed 1738711717
    { "status": "pass", "test_file": "/Stepdown/not_primary_keep/pooled", "seed": "1738711717", "start": 1206.012584, "end": 1206.098426, "elapsed": 0.085842  }

This may help to narrow down a test failure. The failing assert does not identify whether a pooled client was used.