[BUGFIX] Flow Bootstrap now works if PhpUnit is used as Phar file
If PhpUnit was used as a Phar file, the Flow Core Bootstrap failed
during initialization in Testing context because it tried to find
PhpUnit's autoloader. Since autoloading is already in effect when
using the Phar version, a simple check for the existence of a
PhpUnit class is sufficient.
Releases: master, 2.0