[Helpers] Read a file again if the size is 0.

Some tests such as testChildProcessManager::test_createWithEnv()
sometimes fails due to empty of file contents. Although the clear
reason is unknown, we try to read repeatedly in order to avoid
the failure.
[Helpers] Fix a problem in which 10 sec. is wrongly used.
Merge pull request #421 from project-hatohol/retry-assert-file-content

[Helpers] Read a file again if the read size is zero.