feat(mocks): provide access to call options in client tests

[dbolduc]

Fixes #10030

If we think this should be more publicized, say the words and I will add it to our mocking exemplar.

Also, while we are here, put the other mock test in an anonymous namespace.

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This change is 

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (fc30088) 93.75% compared to head (5cda982) 93.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11050      +/-   ##
==========================================
- Coverage   93.75%   93.75%   -0.01%     
==========================================
  Files        1725     1727       +2     
  Lines      155370   155382      +12     
==========================================
+ Hits       145670   145680      +10     
- Misses       9700     9702       +2     

Impacted Files	Coverage Δ
google/cloud/mocks/mock_stream_range_test.cc	100.00% <ø> (ø)
google/cloud/mocks/current_options.h	100.00% <100.00%> (ø)
google/cloud/mocks/current_options_test.cc	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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