Fix docs/google snippet error and add to snippets_test

[ToastCheng]

The PR fixes the md files' snippet errors under docs/google by adding test substitution and MagicMock, also fixing some outdated APIs. On the test side, adding necessary imports for cirq_google to let the tests being able to find necessary variables.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 99.57%. Comparing base (9e2123d) to head (e207a60). :warning: Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7803      +/-   ##
==========================================
- Coverage   99.57%   99.57%   -0.01%     
==========================================
  Files        1102     1102              
  Lines       98529    98641     +112     
==========================================
+ Hits        98109    98220     +111     
- Misses        420      421       +1     

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