run: reject waiting for remain-after-exit services

Reproducer:
  unit=run-wait-rae-$(date +%s)
  sudo timeout 3s systemd-run --wait --remain-after-exit \
      --unit="$unit" /bin/true
  echo $?
  systemctl is-active "$unit.service"

Before, the command timed out with exit status 124 while the service
stayed active. --wait waits for deactivation, but RemainAfterExit=yes
keeps the service active after the command exits.

Follow-up for 2a453c2ee3

test/units/TEST-74-AUX-UTILS.run.sh

@@ -72,6 +72,8 @@ systemctl cat "$UNIT"
 grep -q "^Type=oneshot" "/run/systemd/transient/$UNIT.service"
 systemctl stop "$UNIT"
 (! systemctl cat "$UNIT")
+(! systemd-run --wait --remain-after-exit true)
+(! systemd-run --pipe --remain-after-exit true)
 
 : "Transient service (user daemon)"
 systemd-run --wait --pipe --user --machine=testuser@ \