build: Fix ssh-agent testing

container/devshell/test-ssh-agent

@@ -6,11 +6,11 @@ cd "$(dirname "$0")"
 # currently that is not what this tests: instead it's testing SSH from where this runs into a container, with an agent.
 
 # Test that the local SSH Agent works (on the laptop/build server, where this script runs; NOT in container)
-if [ -z "$SSH_AUTH_SOCK" ]; then
+SSH_AUTH_SOCK=${SSH_AUTH_SOCK:-default_value}
+if [[ -z "$SSH_AUTH_SOCK" || "$SSH_AUTH_SOCK" == "default_value" ]]; then
   echo "Starting SSH Agent..."
   eval "$(ssh-agent -s)"
 fi
-ssh-add -l
 
 # TODO instead of copy/paste from ../sshd/test, share common test harness structure..
 
@@ -21,6 +21,7 @@ sleep 2
 
 rm -f /tmp/test-$$-id_rsa*
 ssh-keygen -f /tmp/test-$$-id_rsa -P "" -C test
+ssh-add /tmp/test-$$-id_rsa
 docker exec test-"$1" /usr/local/sbin/add-uid-key tester /bin/bash "$(cat /tmp/test-$$-id_rsa.pub)"
 
 # NB -A for Agent Forwarding!