reuse system test targets for convergence test

dune/pdelab-systemtesting/poisson/CMakeLists.txt

-#add_dune_system_test(BASENAME poisson INIFILE poisson.mini SOURCE poisson_uniform.cc TARGETS output)
+add_dune_system_test(BASENAME poisson INIFILE poisson.mini SOURCE poisson_uniform.cc TARGETS output)
 
-add_dune_convergence_test(BASENAME poisson_convergence SOURCE poisson_uniform.cc INIFILE poisson_conv.mini OUTPUT_TARGETS output)
+# The following block is supposed to vanish as soon as FS1557 is through!
 foreach(target ${output})
   add_dune_ug_flags(${target})
   add_dune_umfpack_flags(${target})
@@ -8,4 +8,5 @@ foreach(target ${output})
   target_link_libraries(${target} ${DUNE_LIBS})
 endforeach()
 
-#add_dune_convergence_test(BASENAME poisson_convergence TARGET ${output} INIFILE poisson_conv.mini)
\ No newline at end of file
+# reuse the existing executables for a convergence test.
+add_dune_convergence_test(BASENAME poisson_convergence TARGET ${output} INIFILE poisson_conv.mini)
\ No newline at end of file