Force lineSegs output of fov() to be based on chebtech2.

This linear chebfun is constructed by specifying its values at +/- 1.  That
will only work if the underlying tech is chebtech2, since first-kind Chebyshev
points don't include the endpoints.  Thus, we have fov() force the underlying
tech to be chebtech2.

Without this change, the fov() test fails "in first-kind mode".