tests: move viewport creation into helpers

tests: move viewport creation into helpers

There will be a new test program using viewports and would like to share this
bit of code.

There are two behavioral changes:
- Compositor wp_viewporter interface version is no longer checked.
- client_create_viewport() does not leak the viewporter object.

test_viewporter_double_create needs to call bind_to_singleton_global() itself
so that the viewporter object still exists when the error event arrives.
Otherwise error verification fails.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>