test: mask tmpfiles.d file shipped by selinux policy package in containers

This tmpfiles.d wants to write to sysfs, which is read-only in containers,
so systemd-tmpfiles --create fails in TEST-22-TMPFILES when ran in nspawn
if the selinux policy package is instealled. Mask it, as it's not our
config file, we don't need it in the test.

(cherry picked from commit 6fd3496)
(cherry picked from commit 2d975f6)

Author: bluca

test/units/testsuite-22.sh

@@ -6,6 +6,14 @@ set -o pipefail
 # shellcheck source=test/units/test-control.sh
 . "$(dirname "$0")"/test-control.sh
 
+if systemd-detect-virt --quiet --container; then
+    # This comes from the selinux package and tries to write
+    # some files under sysfs, which will be read-only in a container,
+    # so mask it. It's not our tmpfiles.d file anyway.
+    mkdir -p /run/tmpfiles.d/
+    ln -s /dev/null /run/tmpfiles.d/selinux-policy.conf
+fi
+
 run_subtests
 
 touch /testok