test: flush the socket once the triggered unit exits

Since the triggered unit intentionally fails without consuming any data
from the socket, we'd try to trigger it again and again, and we might
try to check the unit state in one of the "in-between" states, failing
the test:

[  165.271698] H testsuite-07.sh[1032]: + systemctl start badbin_assert.socket
[  165.977637] H testsuite-07.sh[1032]: + socat - ABSTRACT-CONNECT:badbin_assert.socket
[  165.983787] H systemd[1]: Cannot find unit for notify message of PID 1039, ignoring.
[  166.817187] H testsuite-07.sh[1032]: + timeout 10 sh -c 'while systemctl is-active badbin_assert.service; do sleep .5; done'
[  167.049218] H testsuite-07.sh[1065]: active
[  167.146854] H systemd[1]: Listening on badbin_assert.socket.
[  167.163473] H systemd[1]: badbin_assert.socket: Incoming traffic
[  167.542626] H systemd[1]: Cannot find unit for notify message of PID 1065, ignoring.
[  167.543437] H (badbin)[1062]: badbin_assert.service: Failed to execute /tmp/badbin: Exec format error
[  167.548346] H systemd[1]: badbin_assert.service: Main process exited, code=exited, status=203/EXEC
[  167.549482] H systemd[1]: badbin_assert.service: Failed with result 'exit-code'.
[  167.561537] H systemd[1]: badbin_assert.socket: Incoming traffic
[  167.933390] H systemd[1]: Started badbin_assert.service.
[  167.950489] H (badbin)[1070]: badbin_assert.service: Failed to execute /tmp/badbin: Exec format error
[  167.956318] H systemd[1]: badbin_assert.service: Main process exited, code=exited, status=203/EXEC
[  167.957173] H systemd[1]: badbin_assert.service: Failed with result 'exit-code'.
[  167.974609] H systemd[1]: badbin_assert.socket: Incoming traffic
[  168.042838] H testsuite-07.sh[1072]: failed
[  168.094431] H testsuite-07.sh[1075]: ++ systemctl show -P ExecMainStatus badbin_assert.service
[  168.704022] H systemd[1]: Started badbin_assert.service.
[  168.778680] H (badbin)[1074]: badbin_assert.service: Failed to execute /tmp/badbin: Exec format error
[  168.826881] H systemd[1]: badbin_assert.service: Main process exited, code=exited, status=203/EXEC
[  168.833825] H systemd[1]: badbin_assert.service: Failed with result 'exit-code'.
[  168.923931] H testsuite-07.sh[1032]: + [[ 0 == 203 ]]
[  168.951492] H systemd[1]: Cannot find unit for notify message of PID 1075, ignoring.
[  168.999862] H testsuite-07.sh[615]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/testsuite-07.issue-30412.sh failed'
[  168.999862] H testsuite-07.sh[615]: Subtest /usr/lib/systemd/tests/testdata/units/testsuite-07.issue-30412.sh failed

Follow-up for 1eeaa93 and 28a2d27.

test/units/testsuite-07.issue-30412.sh

@@ -20,6 +20,7 @@ EOF
 cat >/run/systemd/system/badbin_assert.socket <<EOF
 [Socket]
 ListenStream=@badbin_assert.socket
+FlushPending=yes
 EOF
 
 systemctl daemon-reload