Add test_return_codes check for valid subunit output

This commit adds a check to the test_return_codes module to ensure that
valid subunit is being emitted when we say the command expects subunit
output. This check is accomplished by processing the output stream and
converting it to a list of dicts. Then we check that at least 1 test was
present in the output stream (which should be the case for all the
tests) to ensure it actually is processing the subunit.

This is a follow on unit test for #102

stestr/tests/test_return_codes.py

@@ -12,6 +12,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import functools
+import io
 import os
 import re
 import shutil
@@ -20,6 +22,8 @@
 
 import six
 from six import StringIO
+import subunit as subunit_lib
+import testtools
 
 from stestr.tests import base
 
@@ -67,6 +71,23 @@ def assertRunExit(self, cmd, expected, subunit=False):
             self.assertEqual(p.returncode, expected,
                              "Expected return code: %s doesn't match actual "
                              "return code of: %s" % (expected, p.returncode))
+            output_stream = io.BytesIO(out)
+            stream = subunit_lib.ByteStreamToStreamResult(output_stream)
+            starts = testtools.StreamResult()
+            summary = testtools.StreamSummary()
+            tests = []
+
+            def _add_dict(test):
+                tests.append(test)
+
+            outcomes = testtools.StreamToDict(functools.partial(_add_dict))
+            result = testtools.CopyStreamResult([starts, outcomes, summary])
+            result.startTestRun()
+            try:
+                stream.run(result)
+            finally:
+                result.stopTestRun()
+            self.assertThat(len(tests), testtools.matchers.GreaterThan(0))
 
     def test_parallel_passing(self):
         self.assertRunExit('stestr run passing', 0)