test: don't make the final vacuum multiple of 8M

As this would usually mean we'd nuke everything, since there are two
open journals - system and user - both being 8M in size.

test/units/testsuite-04.sh

@@ -4,7 +4,7 @@ set -eux
 set -o pipefail
 
 # Limit the maximum journal size
-trap "journalctl --rotate --vacuum-size=16M" EXIT
+trap "journalctl --rotate --vacuum-size=20M" EXIT
 
 # shellcheck source=test/units/test-control.sh
 . "$(dirname "$0")"/test-control.sh