test(bigtable): add Options constructor for mock clients

[dbolduc]

Part of the work for #6307

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7232 (6a10fe6) into main (b711471) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7232      +/-   ##
==========================================
- Coverage   93.79%   93.79%   -0.01%     
==========================================
  Files        1338     1338              
  Lines      115145   115148       +3     
==========================================
+ Hits       108000   108002       +2     
- Misses       7145     7146       +1     

Impacted Files	Coverage Δ
google/cloud/bigtable/testing/mock_admin_client.h	63.63% <100.00%> (+0.84%)	⬆️
google/cloud/bigtable/testing/mock_data_client.h	79.16% <100.00%> (+0.90%)	⬆️
...loud/bigtable/testing/mock_instance_admin_client.h	69.56% <100.00%> (+0.67%)	⬆️
google/cloud/grpc_error_delegate.cc	95.83% <0.00%> (-4.17%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.59% <0.00%> (+0.37%)	⬆️

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