hp convergence test

Dear all,

Just a little suggestion to add a simple validation test to the CI.

I propose the hp convergence test of a 2D Helmholtz infinite rectangular waveguide problem excited with its first mode. The current version implements only the oo0 transmission condition and uses a CTest framework similar to examples and tutorials. In the same logic, I added a folder validations. Of course, feel free to change all of the above ;).

This is clearly not a panacea, but this simple test was able to detect the bug found in MR !31 (merged). Yet, convergence errors do not occur at the same place for different runs, in the particular example of MR !31 (merged) [1]. Nonetheless, I believe it might be useful.

The "default" batch takes around 100s on the CI infrastructure of GmshDDM and runs only for master. Once more, feel free to change this if needed.

I welcome of course any comments/thoughts/cup-of-coffee regarding this MR :).

Cheers, Nicolas.

[1] That being said, it is coherent with what I experienced with my own CI.