[1p][niconvection] fix variable name for threshold setting in test

The name given for the threshold setting for the velocity did not coincide with the one
used in the reference solution. However, the threshold used here several orders
of magnitude larger than the maximum occurring velocity. There are actually discrepancies
in velocity of 1% with respect to the reference solution!

test/porousmediumflow/1p/implicit/CMakeLists.txt

@@ -51,7 +51,7 @@ add_dumux_test(test_box1pniconduction test_box1pniconduction test_box1pniconduct
                  --files ${CMAKE_SOURCE_DIR}/test/references/1pniboxconduction-reference.vtu
                          ${CMAKE_CURRENT_BINARY_DIR}/box1pniconduction-00006.vtu
                  --command "${CMAKE_CURRENT_BINARY_DIR}/test_box1pniconduction"
-                 --zeroThreshold {"velocity":1e-8})
+                 --zeroThreshold {"velocity_H2O":1e-8})
 
 add_dumux_test(test_box1pniconvection test_box1pniconvection test_box1pniconvection.cc
                python ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py