apt: add apt selftest to test signed package feeds

apt: add apt selftest to test signed package feeds

Since Gatesgarth apt (1.8.2) has become more strict and doesn’t allow unsigned repositories by default.
Currently when building images this requirement is worked around by using [allow-insecure=yes] and
equivalently when performing selftest.

Patches "gpg-sign: Add parameters to gpg signature function" and "package_manager: sign DEB package feeds"
enable signed DEB package feeds. This patch adds a runtime test for apt derived from the test_testimage_dnf
test. It creates a signed deb package feed, runs a qemu image to install the key and performs some package
management. To be able to install the key the gnupg package is added to the testimage.

Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>