test: Fix mtr-packet tests for Python 3

test: Fix mtr-packet tests for Python 3

The mtr-packet test suite was written to run with Python 2, and lacked
the Unicode to bytes and bytes to Unicode conversions required to
run with Python 3.  This change adds those conversions, and the result
works with either Python 2 or Python 3.

Also, close the subprocess's stdin and stdout explicitly.  Python 3's
unittest complains about unclosed file descriptors otherwise.