Add test for fchmod and fchmodat

[Fabien Siron]

Hello list,

Before trying to apply for GSOC, I think it's a really good idea to make
a first contribution. Thus, it seems that an easy and useful contribution
would be a test. I noticed that a chmod test has been recently added to
the project and I think that complete the *chmod family* test could be
good. But I have two questions:

* Does this choice of test is a good one?
* If yes, do fchmod and fchmodat have to be tested in the same test as
chmod or in a separated one?

Cheers,

--
Fabien Siron