Add test coverage for Authcache Enum API

Oh, fun:

$hex='a0';
$dec='10';
$a[$hex] = 'hex';
$a[$dec] = 'dec';
var_dump($a);

# => array(2) {
#      'a0' =>
#      string(3) "hex"
#      [10] =>
#      string(3) "dec"
#    }

I think the correct way to resolve this is to ensure that authcache_enum_keys() returns an array of strings.