cellular: Migrate network_3GModemControl to new test_environment

BUG=chromium:415293
TEST=Run network_3GModemControl on E362, Gobi3k, hefty, pseudomodem

Change-Id: Id28e86479b7386290a8b06458ccd7f7f4097db97
Reviewed-on: https://chromium-review.googlesource.com/220964
Tested-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
diff --git a/client/cros/cellular/test_environment.py b/client/cros/cellular/test_environment.py
index 93293a2..723343d 100644
--- a/client/cros/cellular/test_environment.py
+++ b/client/cros/cellular/test_environment.py
@@ -134,6 +134,10 @@
     def __exit__(self, exception, value, traceback):
         if self._nested:
             return self._nested.__exit__(exception, value, traceback)
+        self.shill = None
+        self.flim = None
+        self.modem_manager = None
+        self.modem = None
 
 
     def _get_shill_cellular_device_object(self):