tests: add wp_viewporter tests

tests: add wp_viewporter tests

These tests poke the viewporter interface to ensure proper behaviour
from client perspective, without testing the rendering result.

These cases are covered:
- create viewport twice
- source rectangle invalid value errors, and unset
- destination size invalid value errors, and unset
- source causing non-integer destination size
- source inside/outside of buffer with transform, scale
- source outside NULL buffer, then getting real buffer
- source outside NULL buffer with inherited NULL
- set_source, set_destination, and destroy after the wl_surface is
  destroyed

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>