test: move TEST-26-SETENV setup to static files

Also run mask_supporting_services() here.

test/units/testsuite-26.service

@@ -0,0 +1,6 @@
+[Unit]
+Description=TEST-26-SETENV
+
+[Service]
+ExecStart=/usr/lib/systemd/tests/testdata/units/%N.sh
+Type=oneshot

test/units/testsuite-26.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+set -ex
+set -o pipefail
+
+# Make sure PATH is set
+systemctl show-environment | grep -q '^PATH='
+
+# Let's add an entry and override a built-in one
+systemctl set-environment PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/testaddition FOO=BAR
+
+# Check that both are set
+systemctl show-environment | grep -q '^PATH=.*testaddition$'
+systemctl show-environment | grep -q '^FOO=BAR$'
+
+systemctl daemon-reload
+
+# Check again after the reload
+systemctl show-environment | grep -q '^PATH=.*testaddition$'
+systemctl show-environment | grep -q '^FOO=BAR$'
+
+# Drop both
+systemctl unset-environment FOO PATH
+
+# Check that one is gone and the other reverted to the built-in
+! (systemctl show-environment | grep -q '^FOO=$')
+! (systemctl show-environment | grep -q '^PATH=.*testaddition$')
+systemctl show-environment | grep -q '^PATH='
+
+echo OK > /testok
+
+exit 0