clean up test output, and verify that the error message is correct..

loraserv.py

@@ -1,5 +1,6 @@
 import argparse
 import asyncio
+import io
 import multicast
 import sys
 import unittest
@@ -168,10 +169,15 @@ class TestLoraServ(unittest.IsolatedAsyncioTestCase):
 
 	@timeout(2)
 	async def test_argerrors(self):
-		# it'd be nice to silence the usage output here
-		with self.assertRaises(SystemExit) as cm, \
-		    patch.dict(sys.__dict__, dict(argv=[ 'name', ])):
-			await main()
+		with io.StringIO() as fp:
+			with self.assertRaises(SystemExit) as cm, \
+			    patch.dict(sys.__dict__, dict(argv=[ 'name', ],
+			      stderr=fp)):
+				await main()
+
+			errout = fp.getvalue()
+
+		self.assertEqual(errout, 'usage: name [-h] [-a maddr] serdev\nname: error: the following arguments are required: serdev\n')
 
 		self.assertEqual(cm.exception.code, 2)