clustermanager: fix test logic waiting for picker during graceful switch

[easwars]

Fixes #8995

The test was waiting for an event that was not expected to happen, but was waiting for it with the test context instead of a short context. This is the offending line: https://github.com/grpc/grpc-go/blob/master/internal/xds/balancer/clustermanager/clustermanager_test.go#L668-L672

I ended up cleaning the test up, adding comments to make it more readable, and removing a stub policy that was not being used in anyway.

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.53%. Comparing base (c01f4f1) to head (8ae4c83).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9072      +/-   ##
==========================================
- Coverage   80.90%   80.53%   -0.38%     
==========================================
  Files         413      413              
  Lines       33509    33543      +34     
==========================================
- Hits        27111    27013      -98     
- Misses       4299     4315      +16     
- Partials     2099     2215     +116     

see 25 files with indirect coverage changes

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