test_performance: relax network speed to 2 MB/s

At QUARTIQ I am getting 2.4/2.3 MB/s and with single switch at M-Labs we
apparently regularly met 2.2/2.2 MB/s. But with the current multiple
switches and one of them being a problematic switch that triggered #837
it looks like it is a tad slower.

http://buildbot.m-labs.hk/builders/artiq/builds/1818/steps/python_coverage_1/logs/stdio

artiq/test/coredevice/test_performance.py

@@ -43,12 +43,12 @@ def test_host_to_device(self):
         exp = self.create(_Transfer)
         host_to_device_rate = exp.host_to_device()
         print(host_to_device_rate, "B/s")
-        self.assertGreater(host_to_device_rate, 2.2e6)
+        self.assertGreater(host_to_device_rate, 2e6)
 
     @unittest.skipUnless(artiq_low_latency,
                          "timings are dependent on CPU load and network conditions")
     def test_device_to_host(self):
         exp = self.create(_Transfer)
         device_to_host_rate = exp.device_to_host()
         print(device_to_host_rate, "B/s")
-        self.assertGreater(device_to_host_rate, 2.2e6)
+        self.assertGreater(device_to_host_rate, 2e6)