Add unit tests for marshaling code

[zanebeckwith]

The functions in the marshal.c file convert from the C structures to on-the-wire format, and vice-versa. Currently, this code is only tested implicitly, by testing the higher-level functions that use the (un)marshaling code.

As a quick first step to testing this important code, we should add tests that simply create each of the tss2_tpm_types.h structs, marshals each one to a buffer, un-marshals it to a new struct, and checks that the two structs are the same.