[patch v1] add ptmx support to test-container

[Florian Weimer]

* H. J. Lu:

> I think the ptmx test should be done with header files in
> sysdeps/generic and sysdeps/unix/sysv/linux to avoid
> "#ifdef __linux__".

We can move the whole file to sysdeps/unix/sysv/linux and replace it
with a stub that just calls FAIL_UNSUPPORTED.

Thanks,
Florian