tests: Include poll.h rather than sys/poll.h

sys/poll.h is a non-standard location of the poll.h header, and is
incorrect on non-glibc libcs. poll.h, however, is defined in SUS (v2)
and is more portable.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93764
http://pubs.opengroup.org/onlinepubs/007908799/xsh/poll.h.html


Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>