Revert explicit library arguments to dune_add_test()

The lib of the module itself is linked automatically if it was
registered before cmake processes this subdirectory.

dune/solvers/test/CMakeLists.txt

-dune_add_test(SOURCES cgsteptest.cc
-              LINK_LIBRARIES dunesolvers)
-dune_add_test(SOURCES gssteptest.cc
-              LINK_LIBRARIES dunesolvers)
-dune_add_test(SOURCES mmgtest.cc
-              LINK_LIBRARIES dunesolvers)
-dune_add_test(SOURCES multigridtest.cc
-              LINK_LIBRARIES dunesolvers)
-dune_add_test(SOURCES projectedgradienttest.cc
-              LINK_LIBRARIES dunesolvers)
-dune_add_test(SOURCES obstacletnnmgtest.cc
-              LINK_LIBRARIES dunesolvers)
+dune_add_test(SOURCES cgsteptest.cc)
+dune_add_test(SOURCES gssteptest.cc)
+dune_add_test(SOURCES mmgtest.cc)
+dune_add_test(SOURCES multigridtest.cc)
+dune_add_test(SOURCES projectedgradienttest.cc)
+dune_add_test(SOURCES obstacletnnmgtest.cc)
 dune_add_test(SOURCES genericvectortoolstest.cc)
 dune_add_test(SOURCES lowrankoperatortest.cc)
 dune_add_test(SOURCES nulloperatortest.cc)