Add process output for flutter_tester test and unskip (#18868)

packages/flutter_tools/test/integration/flutter_tester_test.dart

@@ -94,15 +94,20 @@ class MyApp extends StatelessWidget {
 }
 ''');
 
+      // Capture process output so that if the process quits we can print the
+      // stdout/stderr in the error.
+      final StringBuffer logs = new StringBuffer();
+      device.getLogReader().logLines.listen(logs.write);
+
       final LaunchResult result = await start(mainPath);
 
       expect(result.started, isTrue);
       expect(result.observatoryUri, isNotNull);
 
       await new Future<void>.delayed(const Duration(seconds: 3));
-      expect(device.isRunning, true);
+      expect(device.isRunning, true, reason: 'Device did not remain running.\n\n$logs'.trim());
 
       expect(await device.stopApp(null), isTrue);
-    }, skip: true);
+    });
   });
 }