Fixing test on GCC4

src/Solvers/petsc_solver_unit_tests.cpp

test/AMG_psi2_0_test_GCC4.vtk

test/AMG_psi2_1_test_GCC4.vtk

test/AMG_psi2_2_test_GCC4.vtk

test/AMG_psi_0_test_GCC4.vtk

test/AMG_psi_1_test_GCC4.vtk

test/AMG_psi_2_test_GCC4.vtk

...lver<double>_lid_driven_cavity_3d_p1_grid_0_test_GCC4.vtk

...lver<double>_lid_driven_cavity_3d_p2_grid_0_test_GCC4.vtk

...lver<double>_lid_driven_cavity_3d_p2_grid_1_test_GCC4.vtk

...lver<double>_lid_driven_cavity_3d_p3_grid_0_test_GCC4.vtk

...lver<double>_lid_driven_cavity_3d_p3_grid_1_test_GCC4.vtk

...lver<double>_lid_driven_cavity_3d_p3_grid_2_test_GCC4.vtk