test(bigtable): log more to deflake a test

[coryan]

This change is 

[codecov]

Codecov Report

Patch coverage: 62.96% and project coverage change: -0.01 ⚠️

Comparison is base (0d0fa69) 93.69% compared to head (4078e40) 93.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12094      +/-   ##
==========================================
- Coverage   93.69%   93.68%   -0.01%     
==========================================
  Files        2012     2012              
  Lines      175703   175713      +10     
==========================================
- Hits       164619   164615       -4     
- Misses      11084    11098      +14     

Impacted Files	Coverage Δ
...cloud/testing_util/check_predicate_becomes_false.h	69.69% <16.66%> (-30.31%)	⬇️
google/cloud/bigtable/polling_policy_test.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/rpc_retry_policy_test.cc	97.97% <100.00%> (ø)
google/cloud/internal/retry_policy_impl_test.cc	100.00% <100.00%> (ø)
google/cloud/polling_policy_test.cc	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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