src/test: add unittest for MemoryTHP=

This checks if the prctl is set correctly when the property name
is passed to systemd-run.

test/units/TEST-07-PID1.thp-disable.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+systemd-run --wait \
+        /usr/lib/systemd/tests/unit-tests/manual/test-thp no-disable
+
+systemd-run --wait \
+        -p MemoryTHP=disable \
+        /usr/lib/systemd/tests/unit-tests/manual/test-thp disable
+
+# The following test will always return 77 if at compile time the kernel version
+# is less than 6.18. If it happens don't let the whole test fail
+set +e
+
+systemd-run --wait \
+        -p MemoryTHP=madvise \
+        /usr/lib/systemd/tests/unit-tests/manual/test-thp madvise
+
+if [ $? -eq 77 ]; then
+        exit 0
+fi
+
+set -e