oeqa/selftest/cve_check: add tests for recipe and image reports

oeqa/selftest/cve_check: add tests for recipe and image reports

Add a test to verify that the JSON reports are generated correctly for
both single recipe builds and image builds.

More tests are needed, but this is better than nothing.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit df0f35555b09c4bc75470eb45ec9c74e6587d460)
Signed-off-by: Steve Sakoman <steve@sakoman.com>