[bugfix,release] Fix bug in graphtest.cc

In commit 3e405a0b a parameter 'eps'
was introduced for testAggregate(). However this parameter is not
supplied in the call (breaking the test) and it is never used
in the method.

(cherry picked from commit 491ca46b)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>