[Go] Add unit testing for Go Petstore API client

[wing328]

Currently, we only have test.go to perform a simple HTTP request.

We want to add more tests with https://golang.org/pkg/testing/ to ensure the Go API client works without issues.

If you would like to contribute this task, please reply to let us know.

[wing328]

To begin with, we want to test the following methods as a starting point:

• AddPet
• GetPetById
• UpdatePetWithForm

[guohuang]

for these tests, do you want to dynamically generate it or you expect static test?

[wing328]

Static test is a good starting point. Later we can add auto-generate test cases similar to PHP, C#, Perl: #2276