How to run External Unittests

From commit log:

       Added ability to store reference images of the unittest in a
       separate folder (outside repository).

       It consists of 3 major parts:

       1) checkQImageExternal() is expected to be used to fetch data from
          external folders only.

       2) KRITA_UNITTESTS_DATA_DIR environment variable is used to search for
          additional data sources

       3) KRITA_WRITE_UNITTESTS=1 together with KRITA_UNITTESTS_DATA_DIR set
          to a path will write the output of the unittest as a reference to
          an external folder.

    The testing images are stored in:
     svn+ssh://svn@svn.kde.org/home/kde/trunk/tests/kritatests