Reusing targets results in addition of too many tests.

In my proof of concept system test I try to reuse targets for convergence tests like this:

add_dune_system_test(BASENAME poisson
                     INIFILE poisson.mini
                     SOURCE poisson_uniform.cc
                     CREATED_TARGETS output
                     SCRIPT ${DUNE_TESTTOOLS_PATH}/python/wrapper/vtkcompare.py)

add_dune_system_test(TARGET ${output}
                     INIFILE poisson_conv.mini
                     SCRIPT ${DUNE_TESTTOOLS_PATH}/python/wrapper/convergencetest.py)

However, crappy things such as this happen:

    Start 1: poisson_ug_2d_Q1_0001
1/6 Test #1: poisson_ug_2d_Q1_0001 ................   Passed    0.16 sec
    Start 2: poisson_yasp_2d_Q1_0000
2/6 Test #2: poisson_yasp_2d_Q1_0000 ..............   Passed    0.09 sec
    Start 3: poisson_ug_2d_Q1_conv_ug_2d_Q1
3/6 Test #3: poisson_ug_2d_Q1_conv_ug_2d_Q1 .......   Passed    0.27 sec
    Start 4: poisson_ug_2d_Q1_conv_yasp_2d_Q1
4/6 Test #4: poisson_ug_2d_Q1_conv_yasp_2d_Q1 .....   Passed    0.29 sec
    Start 5: poisson_yasp_2d_Q1_conv_ug_2d_Q1
5/6 Test #5: poisson_yasp_2d_Q1_conv_ug_2d_Q1 .....   Passed    0.25 sec
    Start 6: poisson_yasp_2d_Q1_conv_yasp_2d_Q1
6/6 Test #6: poisson_yasp_2d_Q1_conv_yasp_2d_Q1 ...   Passed    0.26 sec