Fix capture output when running tests that call the ConanRunner in the conanfile

[czoido]

Changelog: Fix: Fix capture output when running tests that call the ConanRunner in the conanfile.
Docs: omit

When the ConanRunner was called inside the conanfile the output was not correctly captured depending the case, making imposible to make tests that check the self.run method output. This was because _UnbufferedWrite was encapsulating the ConanOutput and checking if the output had to be captured was returning the wrong results:

conan/conans/client/runner.py

Lines 79 to 85 in 558a2ce

	capture_output = log_handler or not self._log_run_to_output or (
	stream_output and isinstance(stream_output._stream, six.StringIO))
	if capture_output:
	proc = Popen(command, shell=isinstance(command, six.string_types), stdout=PIPE,
	stderr=STDOUT, cwd=cwd)
	else:
	proc = Popen(command, shell=isinstance(command, six.string_types), cwd=cwd)

(because the stream_output._stream was not an instance of six.StringIO but of TestBufferConanOutput or ConanOutput)