Fix ResourceWarning from SubunitTestRunner._list()

This commit fixes the ResourceWarning emitted by stestr's
SubunitRunner._list method caused by a leaking file descriptor when that
method exits. The root cause of this was this method was calling
fdopen() internally to open a new descriptor to the user specified
result stream and never closing it when the method returns. The issue
was that the return from that method had a reference to that fd and
closing it would have resulted in potentially unexpected behavior.
However, this code is not needed, it was just ported from subunit's
SubunitTestRunner class when this code was forked and rewritten to use
unittest instead of testtools. The subunit code is used in a broader
context and the fdopen might be needed there. But for stestr, we
always use stdout for the result stream as this only gets called
internally the worker processes to run tests. If we used something other
than stdout the result stream would not get sent to the parent process.
Since we're alwwys using stdout we don't need an fdopen call because we
never will need a fresh fd for it. This commit removes the legacy
baggage causing this ResourceWarning and just interacts with the stream
directly avoiding an additional open that never gets closed.

Related to #320

stestr/subunit_runner/run.py

@@ -71,14 +71,7 @@ def list(self, test, loader=None):
 
     def _list(self, test):
         test_ids, errors = program.list_test(test)
-        try:
-            fileno = self.stream.fileno()
-        except Exception:
-            fileno = None
-        if fileno is not None:
-            stream = os.fdopen(fileno, "wb", 0)
-        else:
-            stream = self.stream
+        stream = self.stream
         result = StreamResultToBytes(stream)
         for test_id in test_ids:
             result.status(test_id=test_id, test_status="exists")