TEST-74-AUX-UTILS: Support credential-provided root SSH public key

When root authorized keys are provided by mkosi they are not
newline-terminated so appending a public key to the file results
in a corrupt key, so just to be safe we add an empty line.

test/units/testsuite-74.ssh.sh

@@ -32,7 +32,9 @@ removesshid() {
 ssh-keygen -N '' -C '' -t rsa -f "$ROOTID"
 
 mkdir -p 0700 /root/.ssh
-cat "$ROOTID".pub >> /root/.ssh/authorized_keys
+# Add a newline in case authorized_keys wasn't terminated correctly.
+echo >>/root/.ssh/authorized_keys
+cat "$ROOTID".pub >>/root/.ssh/authorized_keys
 
 # set root pw to "foo", just to set it to something valid
 # shellcheck disable=SC2016