Update Helmholtz matrix unit test with init_helmholtz.hpp subclass

Here is a summary of the changes:

• Create a tests/init_helmholtz.hpp subclass to reduce code duplication.
• Consistently add test for seg, quad, tri, hex, prism, pyr, and tet.
• Cuda implementation is compared to StdMat implementation.
• StdMat implementation is compared to Nektar++'s implementation.