Save surrogates as-is in log files

When python sees an invalid character in a filename for the current locale,
instead of clobbering it, it saves is as an invalid codepoint called a
surrogate.  We need to explicitly instruct the encoder to write those out
as-is. In the JSON file, we replace them instead to produce valid json.

mesonbuild/mtest.py

@@ -647,8 +647,8 @@ Timeout:            %4d
         self.logfilename = logfile_base + '.txt'
         self.jsonlogfilename = logfile_base + '.json'
 
-        self.jsonlogfile = open(self.jsonlogfilename, 'w', encoding='utf-8')
-        self.logfile = open(self.logfilename, 'w', encoding='utf-8')
+        self.jsonlogfile = open(self.jsonlogfilename, 'w', encoding='utf-8', errors='replace')
+        self.logfile = open(self.logfilename, 'w', encoding='utf-8', errors='surrogateescape')
 
         self.logfile.write('Log of Meson test suite run on %s\n\n'
                            % datetime.datetime.now().isoformat())