oeqa/selftest/wic: Improve assertTrue calls

oeqa/selftest/wic: Improve assertTrue calls

assertTrue is a problematic call use in test cases since when it fails,
you just get an unhelpful "False is not True" message.

Replace some uses with assertIn/assertNotIn which will give more helpful results
and for the rest, add msg entries which given more helpful debugging.

For example, this patch would help debugging of #15176.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 35d4c39e0df1a304f557471151a03d1e4b0f30c7)
Signed-off-by: Steve Sakoman <steve@sakoman.com>