Use names for the plugin that don't refer to content-trust, as that's
not related to this test.
Make it slightly more clear which steps are preparation and which
are the actual test. The test sometimes fails in the preparation
step, and we could consider moving those separate and XFail the
test if the preparation fails;
Stderr: error pushing plugin: failed to do request: Head "https://registry:5000/v2/plugin-content-trust-upgrade/blobs/sha256:af932a31d4df3a2890f900bcf28e16cea87b2b440b8036ba86ab3418f3e50a35": http: server gave HTTP response to HTTPS client
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>