CF unit tests use incorrect dummy buffers

[jphickey]

This issue was imported from the GSFC issue tracking system

Imported from: [GSFCCFS-1780] CF unit tests use incorrect dummy buffers
Originally submitted by: Hickey, Joseph P. (GSFC-582.0)[VANTAGE SYSTEMS INC] on Tue Nov 16 16:59:31 2021

Original Description:
The CF unit tests contain an oft-repeated sequence to initialize a message pointer, for example:

    /* Arrange */
    pdu_r_msg_t dummy_ph;
    int local_result;

    CF_AppData.engine.in.msg = (CFE_SB_Buffer_t*)&dummy_ph;

This sequence is not valid for two reasons:

1. Because the pdu_r_msg_t instance is not aligned appropriately to be cast to a CFE_SB_Buffer_t*. This invalid cast generates a warning on many compilers.

2. Because the pdu_r_msg_t instance does not contain any additional data beyond the pdu_header_t value. Almost all CF calls will read beyond this header, depending on what the function call is, and some will write too. In the case of writing, this results in stack smashing, and the test may segfault/crash.

Recommendation is to create a union for the message buffer, which can address the alignment problem and also be used to reserve some extra space for data beyond the header that many calls do access.

It looks like this was done at one point in the "pdu_t" type (in cfdp.h) but this was commented out. Recommend reinstating this and using it in unit tests as it will be more correct.