oeqa/selftest/signing: New test for Signing packages in the package feeds.

oeqa/selftest/signing: New test for Signing packages in the package feeds.

[YOCTO # 8134] This test verifies features introduced in bug 8134.

It requires as resources the files from meta-selftest/files/signing:
For 'gpg --gen-key' the used input was:
key: RSA
key-size: 2048
key-valid: 0
realname: testuser
email: testuser@email.com
comment: nocomment
passphrase: test123

Signed-off-by: Daniel Istrate <daniel.alexandrux.istrate@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>