Fix test with gcc7

Some tests where not compiling with GCC v.7.

I have done a few quick fix, but the following tests are failing

• test27cells.sh
• test125cellsPerturbed.sh
• test-driver (dirty crash)
• testRiemannHLLC
• testPeriodicBC.sh
• testPeriodicBCPerturbed.sh

On my master and with GCC 5.4, only the two first are failing.

TestPeriodicBC[Perturbed].sh

failing due to a segmentation fault

RiemannHLLC

[05783.2] testRiemannHLLC.c:check_riemann_symmetry():76: Asymmetry in flux solution!
[05782.9] check_riemann_symmetry: Flux solver asymmetric: [7.467e+00,1.018e+02,-5.346e+01,-9.949e+01,1.567e+03] == [-7.467e+00,-1.018e+02,5.346e+01,9.949e+01,-1.567e+03]

FAIL testRiemannHLLC (exit status: 134)

Test Cells

Accuracy error (relative error of about 1e-3)

Driver

../test-driver: line 107: 17857 Aborted (core dumped) "$@" > $log_file 2>&1

I will look to theses bugs, but if you have any suggestions before I start. I will gladly listen to them