test/TEST-18-FAILUREACTION: fix typo to actually run firstphase action

The if test for running the firstphase of the test should be negated
so the firstphase is actually executed.

test/TEST-18-FAILUREACTION/testsuite.sh

@@ -5,7 +5,7 @@ set -o pipefail
 systemd-run --wait -p FailureAction=poweroff true
 ! systemd-run --wait -p SuccessAction=poweroff false
 
-if test -f /firstphase ; then
+if ! test -f /firstphase ; then
     echo OK > /firstphase
     systemd-run --wait -p SuccessAction=reboot true
 else