tests: remove assertion on message for struct packing errors
tests: remove assertion on message for struct packing errors

Since this exception comes out of a lower layer in the interpreter
the error message from CPython differs from that of PyPy resulting
an failures when testing with PyPy.  Since the test doesn't really
add much, it is now removed.

Signed-off-by: Paul Osborne <2d2305038687bbc5c42a93e4e4419307a6b61a9c@digi.com>

rtzoeller/python-suitcase,nateglims/python-suitcase,digidotcom/python-suitcase,digidotcom/python-suitcase,rtzoeller/python-suitcase,nateglims/python-suitcase