tests: improve TestProcess behavior

- Have it actually write to the given stdout handle.
- Return the return code for `poll` instead of returning `None`, so that
the process is observed to terminate.

qubesadmin/tests/__init__.py

@@ -52,7 +52,7 @@ class TestVMCollection(dict):
 
 
 class TestProcess(object):
-    def __init__(self, input_callback=None, stdout=None, stderr=None):
+    def __init__(self, input_callback=None, stdout=None, stderr=None, stdout_data=None):
         self.input_callback = input_callback
         self.got_any_input = False
         self.stdin = io.BytesIO()
@@ -70,6 +70,10 @@ class TestProcess(object):
             self.stderr = io.BytesIO()
         else:
             self.stderr = stderr
+        if stdout_data:
+            self.stdout.write(stdout_data)
+            # Seek to head so that it can be read later
+            self.stdout.seek(0)
         self.returncode = 0
 
     def store_input(self):
@@ -91,7 +95,7 @@ class TestProcess(object):
         return 0
 
     def poll(self):
-        return None
+        return self.returncode
 
 
 class _AssertNotRaisesContext(object):
@@ -167,11 +171,12 @@ class QubesTest(qubesadmin.app.QubesBase):
         #     raise AssertionError('Unexpected service call {!r}'.format(call_key))
         if call_key in self.expected_service_calls:
             kwargs = kwargs.copy()
-            kwargs['stdout'] = io.BytesIO(self.expected_service_calls[call_key])
+            kwargs['stdout_data'] = self.expected_service_calls[call_key]
         return TestProcess(lambda input: self.service_calls.append((dest,
             service, input)),
             stdout=kwargs.get('stdout', None),
             stderr=kwargs.get('stderr', None),
+            stdout_data=kwargs.get('stdout_data', None),
         )