tests: Reshuffle IVI layout tests

Rename the IVI tests to be more consistent with the others, and invert
the naming of plugin/client to make it slightly more clear what's going
to happen. Handle the renaming by using wet_get_binary_path to rewrite
the local binaries.

As a side-effect, weston.ini ivi-shell-user-interface no longer needs to
be given as an absolute path.

Signed-off-by: Daniel Stone <daniels@collabora.com>

v2:

Call ivi-layout.ivi as ivi-layout-test-client.ivi to keep the same name
in both the file and the lookup, so that the module map does not need to
change the name.

Update code comments to reflect the new names.

Rename ivi_layout-test-plugin.c to ivi-layout-test-plugin.c.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>