[PATCH] tst-ttyname: skip the test when /dev/ptmx is not available

[Florian Weimer]

* Dmitry V. Levin:

>> I don't think the glibc test suite is supposed to pass in such an
>> environment.
>
> It used to work perfectly for at least 15 years, and it still works
> when tst-ttyname is fixed.

This just reflects that we did not have sufficient test coverage for
the PTY code.

> btw, as support_become_root is also unavailable in restricted
> environments, an alternative fix for tst-ttyname is to skip the test
> when support_become_root fails.

But that's unrelated.  And in some environments, support_become_root
can fail, but the process was initially sufficiently privileged to run
the test.