test: check if sd-executor doesn't complain during (de)serializing

This was usually caught by "accident" by other tests that parse output
of commands executed by sd-executor, so let's have an explicit check for
this.

test/units/end.sh

@@ -9,5 +9,9 @@ set -o pipefail
 # Here, the redundant '[ ]' in the pattern is required in order not to match the logged command itself.
 (! journalctl -q -o short-monotonic --grep 'Warning: cannot close sd-bus connection[ ].*after fork' >>/failed)
 
+# Check if sd-executor doesn't complain about not being able to (de)serialize stuff
+(! journalctl -q -o short-monotonic --grep "[F]ailed to parse serialized line" >>/failed)
+(! journalctl -q -o short-monotonic --grep "[F]ailed to (de)?serialize \w+" >>/failed)
+
 systemctl poweroff --no-block
 exit 0