I was wrong.  Revert "Single ints can be stored by value"

They need to be pointers so we can do the tests, and so
they can be pointed to specific locations on the card in.

This reverts commit 4416ad97885776a0eb5125f608b7af61a379ae5f.
Add mu_assert_eq testing macro
Make transfer test more comprehensive

Covers the case where only partial transfer of
the input data is possible.
Only update TX buffer if previous data has been read out
Only readout RX buffer if there room for the whole thing