tests: Include sys/select.h

Used in compliance with POSIX 2001/2008

Fixes errors e.g.
error: implicit declaration of function 'select'

and helps with missing definitions of FD_* defines

v2: conditionally include sys/select.h, include in every test where
needed.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com> (v1)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>