selftests: forwarding: Add a fallback cleanup()

Consistent use of defers obviates the need for a separate test-specific
cleanup function -- everything is just taken care of in defers. So in this
patch, introduce a cleanup() helper in the forwarding lib.sh, which calls
just pre_cleanup() and defer_scopes_cleanup(). Selftests are obviously
still free to override the function.

Since pre_cleanup() is too entangled with forwarding-specific minutia, the
function cannot currently be in net/lib.sh.

Signed-off-by: Petr Machata <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/forwarding/lib.sh

···
 	done
 }
 
+cleanup()
+{
+	pre_cleanup
+	defer_scopes_cleanup
+}
+
 multipath_eval()
 {
 	local desc="$1"