Correcting test for Config.class.php

The constant used to load config file i.e CONFIG_FILE was config.inc.php while it did not exist in code base, which was causing two similar test-case with no config-file.
CONFIG_FILE has been replaced with "config.sample.inc.php" which can be tested for.
 
Signed-off-by: A V Minhaz <minhazav@gmail.com>