Update Packet module unit tests

I added tests which make sure that the packet module can handle hex
values which contain upper case and invalid hex digits.  The lack of
these tests was causing less than 100% code coverage of the inlined
hex_convert routines.
Added unit tests for cmd_file module

The code coverage for cmd_file.c, mri.c, and platformMock.cpp is now
100%.  This completes the unit testing for the MRI core sources as they
all now have 100% code coverage.

I corrected some mistakes that I noticed in the comments of cmd_file.c.
I also added a clearExceptionCode() call in HandleFileIOCommand() to
make sure that exception codes didn't leak out.