tests: implement client_destroy()

tests: implement client_destroy()

It turns out that if the client is not explicitly destroyed, it will remain
connected until the compositor shuts down because there is no more a client
process that would terminate.

Usually this is not a problem, but if a test file has multiple screenshooting
tests, the windows from earlier tests in the file will remain on screen. That
is not wanted, hence implement client destruction.

To properly destroy a client, we also need a list of outputs. They used to be
simply leaked. This does not fix wl_registry.global_remove for wl_outputs, that
is left for a time when a test will actually need that.

This patch makes only ivi-shell-app test use the new client_destroy() to show
that it actually works. The added log scopes prove it: destroy requests get
sent. Sprinkling client_destroy() around in all other tests is left for a time
when it is actually necessary.

ivi-shell-app is a nicely simple test doing little else, hence I picked it.

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