kunit: tool: continue past invalid utf-8 output

kunit.py currently crashes and fails to parse kernel output if it's not
fully valid utf-8.

This can come from memory corruption or just inadvertently printing
out binary data as strings.

E.g. adding this line into a kunit test
  pr_info("\x80")
will cause this exception
  UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position
  1961: invalid start byte

We can tell Python how to handle errors, see
https://docs.python.org/3/library/codecs.html#error-handlers



Unfortunately, it doesn't seem like there's a way to specify this in
just one location, so we need to repeat ourselves quite a bit.

Specify `errors='backslashreplace'` so we instead:
* print out the offending byte as '\x80'
* try and continue parsing the output.
  * as long as the TAP lines themselves are valid, we're fine.

Fixed spelling/grammar in commit log:
Shuah Khan <<skhan@linuxfoundation.org>

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>