test_processid: only assert that it works if we expect it to work

Otherwise, this would fail on, for instance, QNX.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=69702
Reviewed-by: Patrick Welche <prlw1@cam.ac.uk>
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>
Tested-by: Patrick Welche <prlw1@cam.ac.uk>