Upgrade test: work against rc, and say at once when there is nothing to upgrade to

Run against rc, the test stopped at a password prompt and then at a
question, each until its 30 minute limit.

sudo asked for a password although the test had written a rule that
lets it through: sudo takes the last rule that matches and reads its
folder in name order, and the rule's name sorted before the install's
own. The release's updater also runs "sudo -v", which asks unless every
rule for the user lets it through. The rule now sorts last, turns the
question off for the user, and the test checks both before going on.

rc holds what stable holds today. The release's updater, run with
nothing to do, ends at a prompt nobody answers. The test now asks the
channel what it offers before it starts, and with nothing newer on offer
it stops in seconds and says so.

The comment shows the two pictures only when both exist.

.github/workflows/upgrade-test.yml

@@ -195,7 +195,7 @@ jobs:
             else
               echo "**The test did not get as far as a report** (job result: $RESULT)."
             fi
-            if [ -n "$PICTURES" ]; then
+            if [ -n "$PICTURES" ] && [ -s evidence/before.png ] && [ -s evidence/after.png ]; then
               echo
               echo "| Before, $FROM | After, on $TO |"
               echo "| --- | --- |"