Uses predefined environment variables in ConfigTest.

I've fixed ConfigTest to use `PHPBREW_ROOT` and `PHPBREW_HOME` defined in phpunit.xml as dummy data. Then, I've added some test cases for methods in ConfigTest which have side effects.
See https://github.com/phpbrew/phpbrew/pull/417#discussion_r21508756 to understand the background of this PR.
I renamed `$oldPHPBrewRoot` to `$previousPhpBrewRoot`.