test: include MAINPID in notify message in TEST-50-DISSECT for notify socket

test: include MAINPID in notify message in TEST-50-DISSECT for notify socket

Copy what systemd-notify does by default by setting it to the PID of the shell,
so that main process tracking works as expected. Also use test -S instead of ls
to check socket.

[   33.980396] (sh)[1024]: run-p1022-i1322.service: Executing: sh -c "echo READY=1 | ncat --unixsock --udp \$NOTIFY_SOCKET --source /run/notify && env"
[   34.138778] systemd[1]: run-p1022-i1322.service: Child 1024 belongs to run-p1022-i1322.service.
[   34.138825] systemd[1]: run-p1022-i1322.service: Main process exited, code=exited, status=0/SUCCESS (success)
[   34.139451] systemd[1]: run-p1022-i1322.service: Failed with result 'protocol'.
[   34.139559] systemd[1]: run-p1022-i1322.service: Service will not restart (restart setting)
[   34.139573] systemd[1]: run-p1022-i1322.service: Changed start -> failed
[   34.139945] systemd[1]: run-p1022-i1322.service: Job 1364 run-p1022-i1322.service/start finished, result=failed

Fixes #35619

Follow-up for 18bb30c3b2ea7f4497edf86414133667b3e155fe