Implements githubTestSetup and use it instead of TestMain
Implements githubTestSetup and use it instead of TestMain

- I want to make GITHUB_TOKEN and GIT_CONFIG empty, and reset these after test
    - because test may be failed if another test uses these envs
- I thought I can do it by defining TestMain function, but it was misunderstood
- So I implements githubTestSetup method which returns function to teardown

mackerelio/mkr,mackerelio/mkr