balancer/pickfirst/pickfirstleaf: fix race condition in tests

[chressie]

An upcoming change (go.dev/cl/653795) in the Go standard library
surfaces race conditions in these tests.

The race comes from the fact that a connectivity.Shutdown is
unconditionally recorded. This commit fixes this by adding a mutex to
guard access to the recorded states.

RELEASE NOTES: N/A

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: chressie (ab24064)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.41%. Comparing base (af07815) to head (ab24064).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8148      +/-   ##
==========================================
+ Coverage   82.30%   82.41%   +0.11%     
==========================================
  Files         392      392              
  Lines       39132    39132              
==========================================
+ Hits        32207    32251      +44     
+ Misses       5603     5571      -32     
+ Partials     1322     1310      -12     

see 20 files with indirect coverage changes

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[arjan-bal]

LGTM