apparmor: test: Remove some casts which are no-longer required

With some of the stricter type checking in KUnit's EXPECT macros
removed, several casts in policy_unpack_test are no longer required.

Remove the unnecessary casts, making the conditions clearer.

Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>