selftest/wic: Fix dependency issue in rawcopy test

The current test code contains a subtle race. It is building an ext4,
then trying to reuse that image in a wic build for the same recipe.
Whilst this happened to work, the code is within its rights to clean
up the output before the wic code runs.

Avoid this by using separate image targets instead.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>