Respect the provided encoding

UTF-8 is great but if we're claiming the encoding is something else everywhere
else we can't just make it UTF-8 here.

src/allmydata/test/common_util.py

@@ -101,11 +101,11 @@ def run_cli_unicode(verb, argv, nodeargs=None, stdin=None, encoding=None):
         argv=argv,
     )
     d = run_cli_bytes(
-        verb.encode("utf-8"),
-        nodeargs=list(arg.encode("utf-8") for arg in nodeargs),
+        verb.encode(encoding),
+        nodeargs=list(arg.encode(encoding) for arg in nodeargs),
         stdin=stdin,
         encoding=encoding,
-        *list(arg.encode("utf-8") for arg in argv)
+        *list(arg.encode(encoding) for arg in argv)
     )
     def maybe_decode(result):
         code, stdout, stderr = result

src/allmydata/util/encodingutil.py

@@ -259,7 +259,7 @@ def quote_output_u(*args, **kwargs):
     result = quote_output(*args, **kwargs)
     if isinstance(result, unicode):
         return result
-    return result.decode("utf-8")
+    return result.decode(kwargs.get("encoding", None) or io_encoding)
 
 
 def quote_output(s, quotemarks=True, quote_newlines=None, encoding=None):