Attempt to deflake the consistenthashing tests

[jrockway]

I can get this to fail when I run 64 copies of the tests simultaneously. While I was doing that I noticed that the etcd logs were complaining about some operations being too slow. Since no CPU was being used while this was going on, I assumed it was a disk problem. Disabling fsync for etcd made the parallel runs all complete in a max of 30 seconds or average of 7 seconds, down from 600 seconds and timing out. So I think this is the magic sauce that was needed. It probably helps other tests, since pretty much everything eventually depends on testetcd.

While working on fixing this test, I noticed that the Loki tests are pretty flaky. That is fixed in #9945.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 57.98%. Comparing base (2463e32) to head (9a41651).

Files	Patch %	Lines
src/internal/pachd/testpachd.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9943      +/-   ##
==========================================
- Coverage   58.15%   57.98%   -0.18%     
==========================================
  Files         610      610              
  Lines       74140    74142       +2     
==========================================
- Hits        43114    42988     -126     
- Misses      30477    30597     +120     
- Partials      549      557       +8     

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