Clean the FeatureContext a bit

While creating https://github.com/Behat/Symfony2Extension/pull/49, I spotted some places which could be cleaned in the FeaturesContext:

- no need to create folders which were used by the Behat 1.x closure step definitions, as they are gone
- the cwd of the main process should not be modified to run Behat in a temporary folder

I also fixed a small issue which appeared for me on windows on one of the tests (the "sometimes" mentioned in the commit message was reproduceable: it only impacted ``features/init.feature:6`` but all the time after the cleanup while never before). But getting the realpath here should not hurt (it should always be an existing folder anyway)