[bugfix][test] Fix CmakeList

I don't know how, but I really srewed up this file before committing.
Now it work's as expected.

dune/solvers/test/CMakeLists.txt

-dune_add_test(SOURCES cgssteptest.cc)
-dune_add_test(SOURCES gsteptest.cc)
+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 obstacletnnmgtest.cc)
-
 dune_add_test(SOURCES genericvectortoolstest.cc)
 dune_add_test(SOURCES lowrankoperatortest.cc)
 dune_add_test(SOURCES nulloperatortest.cc)
 dune_add_test(SOURCES sumoperatortest.cc)
 
+if(HAVE_IPOPT)
+    dune_add_test(SOURCES quadraticipoptsolvertest.cc)
+endif()
 if(SuiteSparse_UMFPACK_FOUND)
   dune_add_test(SOURCES umfpacksolvertest.cc)
   add_dune_suitesparse_flags(umfpacksolvertest)