improve test coverage

[WindzCUHK]

Some of the production code is difficult to test. Add unit tests need additional refactoring.

• main.go
  • cannot run unit test without Athenz API mock
  • code does not have unit test

    • authorization-proxy/main.go

      Lines 130 to 132 in 279622e

      	if !isSignal || len(errs) != 1 || errs[0] != ctx.Err() {
      	return errs
      	}

    • authorization-proxy/main.go

      Lines 181 to 182 in 279622e

      	glg.Info("authorization proxy main process shutdown success")
      	os.Exit(1)