cleanup(mixin): add one API test case for location mixin

[cuiy0006]

In this PR, I add one API test case(Locations - GetLocation) for review.

After it is merged, I will add one API test case for each of LRO and IAM.

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This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.61702% with 156 lines in your changes missing coverage. Please review.

Project coverage is 93.20%. Comparing base (58a05d0) to head (addd368). Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...v1/internal/golden_kitchen_sink_connection_impl.cc	0.00%	9 Missing :warning:
...ternal/golden_kitchen_sink_rest_connection_impl.cc	0.00%	9 Missing :warning:
.../v1/internal/golden_thing_admin_connection_impl.cc	0.00%	9 Missing :warning:
...nternal/golden_thing_admin_rest_connection_impl.cc	0.00%	9 Missing :warning:
...sts/golden/v1/internal/golden_kitchen_sink_stub.cc	0.00%	7 Missing :warning:
...en/v1/internal/golden_kitchen_sink_tracing_stub.cc	0.00%	7 Missing :warning:
...den/v1/internal/golden_thing_admin_tracing_stub.cc	0.00%	7 Missing :warning:
...olden/v1/internal/golden_kitchen_sink_rest_stub.cc	0.00%	6 Missing :warning:
...golden/v1/internal/golden_thing_admin_rest_stub.cc	0.00%	6 Missing :warning:
...ation_tests/tests/golden_kitchen_sink_stub_test.cc	78.57%	6 Missing :warning:
... and 28 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #14813 +/- ## ========================================== - Coverage 93.27% 93.20% -0.07% ========================================== Files 2319 2319 Lines 208292 208514 +222 ========================================== + Hits 194285 194349 +64 - Misses 14007 14165 +158 ```

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