[testMain] Don't open unused pipes.

The pipes has been leaked. Becuase they are not used, we don't
open the pipe to a child process.
[testMain] Use a more simple API.

When the pipe is not opened, g_sapwn_async() is sutaible, which
makes code more simple.
Merge pull request #294 from project-hatohol/dont-open-unused-pipe

[testMain] Don't open unused pipes.