extract and split integration tests

[vvatanabe]

This pull request is just a refactoring of the test.

Details:

• Moved test cases other than integration tests in api_test.go to a suitable test file.
  • Moved the test cases for the UnmarshalJSON function of APIError to error_test.go and consolidated them into table tests.
  • numTokens function is not used in the integration test, so it was moved to openai_test.go, which consolidates commonly used functions.
  • Split the test scope of TestRequestError test function.
    • Extracted only mock test of ListEngines API and moved it to engines_test.go.
    • Moved RequestError unmarshalling test to error_test.go.
• Renamed api_test.go to api_integration_test.go to make it easier to understand that it is an integration test.

[codecov]

Codecov Report

Merging #389 (7fc065e) into master (e49d771) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #389   +/-   ##
=======================================
  Coverage   95.26%   95.26%           
=======================================
  Files          17       17           
  Lines         676      676           
=======================================
  Hits          644      644           
  Misses         22       22           
  Partials       10       10           

[vvatanabe]

@sashabaranov Please check it. (just a refactoring of the test.)

[sashabaranov]

Thank you so much 🙌🏻 That was long overdue!