The code was using Wic instead of WicTestCase for image_is_ready and
native_sysroot which was confusing some tests. Fix image_is_ready.
For native_sysroot, convert the only user to just run the call itself
as there is no point in executing this for all the other tests which
don't use the value.
Whilst here, move _get_image_env_path to the base class alongside
its parent variabe and fix it to use WicTestCase.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
