test/x11comp: don't hang if Xvfb is not available

If Xvfb is not present, posix_spawn still forks, but the child fails.
In that case, since we left the write fd of the pipe open in the parent,
we just kept waiting on the read() without noticing that the other side
is dead.

Signed-off-by: Ran Benita <ran234@gmail.com>