Triangle Mesh weirdness causes test fail

https://github.com/firedrakeproject/firedrake/blob/master/tests/test_facets.py

The last test is failing for me.  It tests an integral on internal facets; however, since there should not be any internal facets, the integral should be exactly 0.

UnitTriangleMesh() is apparantly supposed to have vertices at (0,0), (0,1), (1,0).  However, an extra vertex is being created at (0.5,0.5), and the triangle is being divided into two smaller triangles, creating an internal facet.  Integrating x[0] along this facet gives 0.5*(0.5^2) * sqrt(2), which is the result being returned by the integral.

@wence- thoughts?