test/py: Fix unicode handling for log filtering

test/py: Fix unicode handling for log filtering

At present the unicode filtering seems to get confused at times with
this error:

  UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position
     32: ordinal not in range(128)

It seems to be due to self._nonprint being interpreted as UTF-8. Fix it
by using ordinals instead of characters, changing the string to set.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>