[cleanup][cmakelists] reuse executable in 2p2c and 2pncmin

test/porousmediumflow/2p2c/implicit/CMakeLists.txt

 add_subdirectory(mpnccomparison)
 add_input_file_links()
 
-# isothermal tests without caching
-add_executable(test_2p2c_box EXCLUDE_FROM_ALL test_2p2c_fv.cc)
-target_compile_definitions(test_2p2c_box PUBLIC TYPETAG=InjectionBoxTypeTag ENABLECACHING=0)
-
 dune_add_test(NAME test_2p2c_box
-              TARGET test_2p2c_box
+              SOURCES test_2p2c_fv.cc
+              COMPILE_DEFINITIONS TYPETAG=InjectionBoxTypeTag ENABLECACHING=0
               COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
               CMD_ARGS --script fuzzy
                        --files ${CMAKE_SOURCE_DIR}/test/references/injectionbox-reference.vtu

test/porousmediumflow/2pncmin/implicit/CMakeLists.txt

 dune_symlink_to_source_files(FILES test_2pncmin.input)
 
-add_executable(test_2pncmin_box EXCLUDE_FROM_ALL test_2pncmin_fv.cc)
-target_compile_definitions(test_2pncmin_box PUBLIC TYPETAG=DissolutionBoxTypeTag)
-
 # isothermal tests
 dune_add_test(NAME test_2pncmin_box
-              TARGET test_2pncmin_box
+              SOURCES test_2pncmin_fv.cc
+              COMPILE_DEFINITIONS TYPETAG=DissolutionBoxTypeTag
               COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
               CMD_ARGS      --script fuzzy
                             --files ${CMAKE_SOURCE_DIR}/test/references/saltflushbox2pncmin-reference.vtu