Ah, this might be a difference of idiom. I would expect that assertion to print output like this:

  FAILED: Expected RESPONSE_NO_DATA, got RESPONSE_INVALID_ARG
  In /src/foo/ApiTest.cpp:123

For non-enumerated integers, the output would be something like:

  FAILED: Expected ResponseCode(200), got ResponseCode(404)
  In /src/foo/ApiTest.cpp:123

If values are formatted with some amount of self-description, then printing the name of the variable being evaluated is not usually informative.