tests: resolve failure related to unpacking dictionary for protobuf messages

[parthea]

This PR resolves the following failure in #17245 (build log)

==================================== ERRORS ====================================
_ ERROR collecting tests/unit/gapic/bigtable_admin_v2/test_bigtable_instance_admin.py _
tests/unit/gapic/bigtable_admin_v2/test_bigtable_instance_admin.py:7259: in <module>
    iam_policy_pb2.GetIamPolicyRequest({}),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   TypeError: _AddInitMethod.<locals>.init() takes 1 positional argument but 2 were given
_ ERROR collecting tests/unit/gapic/bigtable_admin_v2/test_bigtable_table_admin.py _
tests/unit/gapic/bigtable_admin_v2/test_bigtable_table_admin.py:11222: in <module>
    iam_policy_pb2.GetIamPolicyRequest({}),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   TypeError: _AddInitMethod.<locals>.init() takes 1 positional argument but 2 were given

To resolve the issue, this PR adds ** to unpack fields as keyword arguments, maintaining compatibility with both proto-plus and pure protobuf messages.

[gemini-code-assist]

Code Review

This pull request introduces two new protocol buffer fragment files for testing: locations.proto, which defines a standard Locations service with its request and response messages, and test_protobuf_type.proto, which defines a MockLocationService utilizing the types from locations.proto. There are no review comments, so I have no feedback to provide.