Integration tests: 5min instead of 3min timeout

Recently, mostly on CircleCI, the CI test suite runs into
timeouts, often leading to subsequent "Dirty Reactor" errors.

Increasing the timeout makes this happen less often.

Refs ticket: 4085, 4022, 2994

src/allmydata/test/test_system.py

@@ -115,7 +115,7 @@ class CountingDataUploadable(upload.Data):
 class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
     """Foolscap integration-y tests."""
     FORCE_FOOLSCAP_FOR_STORAGE = True
-    timeout = 180
+    timeout = 300
 
     @property
     def basedir(self):