Goal: 

Improve existing test_gpg_keys_rest [1] so that the `packages_key` part of GPG test asserts that the key is a valid GPG key,

Add a test for an invalid key [2]

Add a test for the meta key [3]

Remove this redundant line:

2. Check introspection with bad gpg key/?

Acceptance Criteria:

Use of invalid key results in an error

Meta key test passes and suitable asserts are made

[1] Existing `packages_key` test : https://gitlab.cee.redhat.com/insights-qe/iqe-content-sources-plugin/-/blob/master/iqe_content_sources/tests/test_repository_api_only.py?ref_type=heads#L69

[2] https://stephenw.fedorapeople.org/fakerepos/gpgkeys/RPM-GPG-KEY-dummy-packages-invalid-key.gpg

[3] Blocker: [API] Valid meta key shows 'Error loading GPG Key: no gpg key was found.'