scripts/pylib/pytest-twister-harness: Do not fail on Unicode garbage

If pylib twister harness reads bytes that can't be decoded to Unicode,
it would raise an exception at a separate "serial reader" process, that
would fail without any warning.

This patch uses "errors='replace'" parameter for Python str.decode()
method, so that this doesn't happen.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>