[Golang][client] Add test for client.StoreApi.PlaceOrder()

[grokify]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Add passing test for Petstore POST /v2/store/order - client.StoreApi.PlaceOrder().

• Uses same values as default Petstore UI
• Uses UTC "Z" timezone. Local offset, e.g. -07:00, returns 500 error from Petstore API server. This type of Local Offset is specified in RFC-3339

@antihax
@bvwells

[wing328]

Tests passed locally and in CircleCI as well: https://circleci.com/gh/OpenAPITools/openapi-generator/681.

Thanks for the enhancement!