Add convergence test for ElementLineHermite

[kinnala]

While reading #384 , I realized there is no convergence test for this element in

https://github.com/kinnala/scikit-fem/blob/master/tests/test_convergence_h2.py

It's easy enough to solve u''''(x)=f analytically in different cases, so it should be possible to create one which estimates the convergence rate in the energy norm.