test: add integration test for Type=exec robustness

This tests for #30799, so that it does not return.

test/units/testsuite-07.type-exec-parallel.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+# Make sure that we never mistake a process starting but failing quickly for a process failing to start, with Type=exec.
+# See https://github.com/systemd/systemd/pull/30799
+
+seq 25 | xargs -n 1 -P 0 systemd-run -p Type=exec /bin/false