[test] re-enable compiling tests for eigen

The FindEigen script's variable name had changed so these tests haven't
beeen built in a while.
Also disables trying to compile eingemmappeddense for complex fields.

dune/xt/la/test/matrices.mini

@@ -3,4 +3,4 @@ __local.matrix_eigen = EigenRowMajorSparseMatrix, EigenDenseMatrix | expand
 __local.matrix_istl = IstlRowMajorSparseMatrix
 matrix = {__local.matrix_common}, {__local.matrix_eigen}, {__local.matrix_istl} | expand types
 
-1, EIGEN_FOUND, dune-istl_FOUND | expand types | cmake_guard
+1, EIGEN3_FOUND, dune-istl_FOUND | expand types | cmake_guard

dune/xt/la/test/vectors.mini

@@ -6,4 +6,6 @@ __local.vector_eigen = EigenDenseVector, EigenMappedDenseVector | expand
 __local.vector_istl = IstlDenseVector
 vector = {__local.vector_common}, {__local.vector_eigen}, {__local.vector_istl} | expand types
 
-1, EIGEN_FOUND, dune-istl_FOUND | expand types | cmake_guard
+1, EIGEN3_FOUND, dune-istl_FOUND | expand types | cmake_guard
+
+{fieldtype} == std::complex<double> and {__local.vector_eigen} ==  EigenMappedDenseVector | exclude