Link loopsolver tests to the library

Tests that make use of a LoopSolver now need to be linked to
libdunesolvers.

dune/solvers/test/CMakeLists.txt

-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 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 genericvectortoolstest.cc)
 dune_add_test(SOURCES lowrankoperatortest.cc)
 dune_add_test(SOURCES nulloperatortest.cc)

[graeser]

They should be linked automatically, because dune_add_test() calls one of the add-all-flags-magic functions. This works e.g. in dune-tnnmg. The proper fix will be to create the lib before we include the subdirectories.

[graeser]

I implemented the proposed fix in 7dc3a9e5 and reverted the explicit LINK_LIBRARIES statements.