tests: try to collect qvm-open-in-dvm output if no editor window is shown

Try to collect more details about why the test failed. This will help
only if qvm-open-in-dvm exist early. On the other hand, if it hang, or
remote side fails to find the right editor (which results in GUI error
message), this change will not provide any more details.

qubes/tests/integ/dispvm.py

@@ -253,12 +253,24 @@ class TC_20_DispVMMixin(object):
             self.testvm1.run_for_stdio("echo test1 > /home/user/test.txt"))
 
         p = self.loop.run_until_complete(
-            self.testvm1.run("qvm-open-in-dvm /home/user/test.txt"))
+            self.testvm1.run("qvm-open-in-dvm /home/user/test.txt",
+            stdout=subprocess.PIPE, stderr=subprocess.STDOUT))
 
         # if first 5 windows isn't expected editor, there is no hope
         winid = None
         for _ in range(5):
-            winid = self.wait_for_window('disp[0-9]*', search_class=True)
+            try:
+                winid = self.wait_for_window('disp[0-9]*', search_class=True)
+            except Exception as e:
+                try:
+                    self.loop.run_until_complete(asyncio.wait_for(p.wait(), 1))
+                except asyncio.TimeoutError:
+                    raise e
+                else:
+                    stdout = self.loop.run_until_complete(p.stdout.read())
+                    self.fail(
+                        'qvm-open-in-dvm exited prematurely with {}: {}'.format(
+                            p.returncode, stdout))
             # get window title
             (window_title, _) = subprocess.Popen(
                 ['xdotool', 'getwindowname', winid], stdout=subprocess.PIPE). \
@@ -278,7 +290,7 @@ class TC_20_DispVMMixin(object):
 
         time.sleep(0.5)
         self._handle_editor(winid)
-        self.loop.run_until_complete(p.wait())
+        self.loop.run_until_complete(p.communicate())
         (test_txt_content, _) = self.loop.run_until_complete(
             self.testvm1.run_for_stdio("cat /home/user/test.txt"))
         # Drop BOM if added by editor