[LNT] Python 3 support: encode LNT report format in UTF-8

[thopre]

Summary

Change encoding of LNT report format to be a UTF-8 JSON file since that
is the only encoding the json module can produce in Python 3. While this
is an incompatible change of the output, the existing import of LNT
report format already expects UTF-8 JSON. Therefore this fixes an import
issue for format produced from there and does not change anything for
already produced format which could not be imported and still cannot be
imported.

[hubert.reinterpretcast]

LGTM.

lnt/trunk/lnt/testing/__init__.py

def render(self, indent=4):
         # Note that we specifically override the encoding to avoid the
         # possibility of encoding errors. Clients which care about the
         # text encoding should supply unicode string objects.
         if self.report_version == 2:
             return json.dumps({'format_version': str(self.report_version),
                                'machine': self.machine.render(),
                                'run': self.run.render(),
                                'tests': [t.render() for t in self.tests]},
-                              sort_keys=True, indent=indent, encoding='latin-1')
+                              sort_keys=True, indent=indent)
         else:
             return json.dumps({'Machine': self.machine.render(),
                                'Run': self.run.render(),
                                'Tests': [t.render() for t in self.tests]},
-                              sort_keys=True, indent=indent, encoding='latin-1')
+                              sort_keys=True, indent=indent)
 
 
 class Machine:
     """Information on the machine the test was run on.
 
     The info dictionary can be used to describe additional information
     about the machine, for example the hardware resources or the
     operating environment.