testsuite-17.11.sh: create all files in a temporary directory

Make sure the test would not collide with anything else by moving
all files it created into a temporary directory.

test/units/testsuite-17.11.sh

@@ -5,6 +5,16 @@ set -o pipefail
 
 # Test for udevadm verify.
 
+cleanup() {
+        cd /
+        rm -rf "${workdir}"
+        workdir=
+}
+
+workdir="$(mktemp -d)"
+trap cleanup EXIT
+cd "${workdir}"
+
 udevadm verify -h
 udevadm verify --help
 udevadm verify -V