Solved by adding below line to Calculatortest.php
file.
require_once dirname(__FILE__) . '/../Calculator.php';
The test class is placed in a subdirectory, so I needed to declare the path relative to the Calculator.php.
Also, I had to change php.ini file so that errors are displayed in case there are.
I set following variables in php.ini.
error_reporting = E_ALL | E_STRICT
display_errors = On