feat: add smoketest for enum values handling in compute

[viacheslav-rostovtsev]

This test verifies that an unknown enum value in proto JSON is being parsed into a label for the default enum value (0).
In proto3 there is always a enum member with a value 0; in Compute specifically a 0-valued member is always a nonfunctional one, called UNDEFINED_<ENUM_NAME>.
For this specific example, for a enum State, it's UNDEFINED_STATE, so an unknown enum value in proto JSON is parsed into a label :UNDEFINED_STATE.

[quartzmo]

It looks to me like this should run as a unit test, not a smoke test. Can you add handwritten unit tests for this library? I copy/pasted just the test (without setup) to test/google/cloud/compute/v1/accelerator_types_test.rb and got this result:

  1) Error:
Google::Cloud::Compute::V1::AcceleratorTypes::ClientTest#test_parse_unknown_enum_value:
Google::Protobuf::ParseError: Error occurred during parsing: Error parsing JSON @1:52: Unknown enumerator: 'THIS_VALUE_DOES_NOT_EXIST'
    /Users/quartzmo/code/google/codez/google-cloud-ruby/google-cloud-compute-v1/test/google/cloud/compute/v1/accelerator_types_test.rb:74:in `decode_json'
    /Users/quartzmo/code/google/codez/google-cloud-ruby/google-cloud-compute-v1/test/google/cloud/compute/v1/accelerator_types_test.rb:74:in `test_parse_unknown_enum_value'

[viacheslav-rostovtsev]

It looks to me like this should run as a unit test, not a smoke test. Can you add handwritten unit tests for this library?

Rewritten as a unit test.

[dazuma]

test(compute-v1): add a test for enum values handling