make test smaller and faster.

[[Imported from SVN: r1136]]

dune/istl/paamg/test/amgtest.cc

@@ -127,7 +127,7 @@ void testAMG(int N, int coarsenTarget, int ml)
 int main(int argc, char** argv)
 {
 
-  int N=500;
+  int N=100;
   int coarsenTarget=1200;
   int ml=10;