feat(bigtable): introduce MockDataConnection and MakeRowReader

[dbolduc]

Part of the work for #8860

this moves our two mock/test classes out of the internal namespace.

We also drop "Test" from the helper name to be consistent with spanner_mocks::MakeRow(...):

google-cloud-cpp/google/cloud/spanner/mocks/row.h

Line 42 in 0673b95

inline spanner::Row MakeRow(

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[google-cloud-cpp-bot]

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Codecov Report

Merging #9335 (0673b95) into main (0673b95) will not change coverage.
The diff coverage is n/a.

❗ Current head 0673b95 differs from pull request most recent head 051c149. Consider uploading reports for the commit 051c149 to get more accurate results

@@           Coverage Diff           @@
##             main    #9335   +/-   ##
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  Coverage   94.66%   94.66%           
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  Files        1470     1470           
  Lines      134793   134793           
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  Hits       127603   127603           
  Misses       7190     7190           

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