test: Fix unit test logging with python3

A logfile must be opened as a text file and encoding is specified.
Then it can accept a string. Current behaviour we get,

 Message: 'ok 2834 sequential/test-worker-prof'
 Arguments: ()
 --- Logging error ---
 Traceback (most recent call last):
   File "/usr/lib64/python3.7/logging/__init__.py", line 1037, in emit
     stream.write(msg + self.terminator)
 TypeError: a bytes-like object is required, not 'str'
 Call stack:
   File "tools/test.py", line 1734, in <module>
     sys.exit(Main())
   File "tools/test.py", line 1710, in Main
     if RunTestCases(cases_to_run, options.progress, options.j, options.flaky_tests):
   File "tools/test.py", line 933, in RunTestCases
     return progress.Run(tasks)
   File "tools/test.py", line 141, in Run
     self.RunSingle(False, 0)
   File "tools/test.py", line 202, in RunSingle
     self.HasRun(output)
   File "tools/test.py", line 365, in HasRun
     logger.info('  ---')

tools/test.py

@@ -1534,7 +1534,7 @@ def Main():
   logger.addHandler(ch)
   logger.setLevel(logging.INFO)
   if options.logfile:
-    fh = logging.FileHandler(options.logfile, mode='wb')
+    fh = logging.FileHandler(options.logfile, encoding='utf-8', mode='w')
     logger.addHandler(fh)
 
   workspace = abspath(join(dirname(sys.argv[0]), '..'))