Unify PHP visibility for all test methods

At the moment most of the test methods are public.

#!/usr/bin/env bash

echo -n 'Public methods: '
grep -r '^ *public function test[A-Z]' . | wc -l
echo -n 'Public methods (PHP4 style): '
grep -r '^ *function test[A-Z]' . | wc -l
echo -n 'Protected methods: '
grep -r '^ *protected function test[A-Z]' . | wc -l

Public methods: 4117
Public methods (PHP4 style): 1379
Protected methods: 9