Update BwdTrans unit test with init_bwdtransfields.hpp subclass

Here is a summary of the changes:

• Reactivate test_bwdtrans.cpp.
• Create a tests/init_bwdtransfields.hpp subclass to reduce code duplication.
• Consistently add test for seg, quad, tri, hex, prism, pyr, and tet.
• Cuda and matrix-free implementations are compared to StdMat implementation.
• StdMat implementation is compared to Nektar++'s implementation.
• Update unit test with padding provision