Fix stderr file creation and test for it going forwards (#495)

* fix stderr file creation and test for it going forwards

* consolidate tests

* clean up imports

manticore/core/workspace.py

@@ -534,7 +534,7 @@ class ManticoreOutput(object):
                 fd.write('{SolverException}')
 
         with self._named_stream('stdout') as _out:
-            with self._named_stream('stdout') as _err:
+            with self._named_stream('stderr') as _err:
                 with self._named_stream('stdin') as _in:
                     with self._named_stream('net') as _net:
                         for name, fd, data in state.platform.syscall_trace:

tests/test_workspace.py

@@ -104,6 +104,7 @@ class StateTest(unittest.TestCase):
         self.assertIn('trace', keys)
         self.assertIn('syscalls', keys)
         self.assertIn('stdout', keys)
+        self.assertIn('stderr', keys)
         self.assertIn('stdin', keys)
         self.assertIn('messages', keys)
         self.assertIn('txt', keys)