oeqa/selftest/bblayers: Add test case for bitbake-layers layerindex-show-depends

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 80090c31)
Signed-off-by: Steve Sakoman <steve@sakoman.com>