Add some simple gmsh reading tests

[MTCam]

This change set adds a few configurations of a simple 3x3 tensor product element mesh generated by gmsh in order to test that 2D gmsh reading is OK. 4 basic tests are added, but in the end we probably don't need them all.

3x3: positively oriented (counter-clockwise element connectivities)
3x3_twisted: positively oriented, but turned so that the el conn doesn't always start with the lower left vertex
3x3_minus: negatively oriented (clockwise element connectivities)
3x3_bound: negatively oriented w/domain boundaries

The two negatively oriented meshes trigger a re-read with the force_positive_orientation=True option set - just to make sure that it works. The final mesh with the domain boundaries currently fails because the force_positive_orientation=True option loses boundary information.

CC: @inducer, @anderson2981