Improved test_triinterp_colinear

This is a one-line change to one of the triangular grid tests.

The various triangular grid functions do not support triangulations containing triangles with colinear points, i.e. zero area.  However, some effort has been made to deal with simple situations containing colinear point triangles.  One of the tests for these "not supported but we try to deal with" situations was overenthusiastic, testing a case that was OK on some but not all compilers.