Stop using deprecated trio.testing.RaisesGroup.

See python-trio/trio#3326.

Apparently Trio maintainers didn't consider the possibility that
others wouldn't be using pytest nor want to add it as a dependency
just for to get this testing utility.

Author: aaugustin

tests/trio/test_messages.py

@@ -508,31 +508,35 @@ async def test_close_is_idempotent(self):
 
     async def test_get_fails_when_get_is_running(self):
         """get cannot be called concurrently."""
-        with trio.testing.RaisesGroup(ConcurrencyError):
+        with self.assertRaises(ExceptionGroup) as raised:
             async with trio.open_nursery() as nursery:
                 nursery.start_soon(self.assembler.get)
                 nursery.start_soon(self.assembler.get)
+        self.assertIsInstance(raised.exception.exceptions[0], ConcurrencyError)
 
     async def test_get_fails_when_get_iter_is_running(self):
         """get cannot be called concurrently with get_iter."""
-        with trio.testing.RaisesGroup(ConcurrencyError):
+        with self.assertRaises(ExceptionGroup) as raised:
             async with trio.open_nursery() as nursery:
                 nursery.start_soon(lambda: alist(self.assembler.get_iter()))
                 nursery.start_soon(self.assembler.get)
+        self.assertIsInstance(raised.exception.exceptions[0], ConcurrencyError)
 
     async def test_get_iter_fails_when_get_is_running(self):
         """get_iter cannot be called concurrently with get."""
-        with trio.testing.RaisesGroup(ConcurrencyError):
+        with self.assertRaises(ExceptionGroup) as raised:
             async with trio.open_nursery() as nursery:
                 nursery.start_soon(self.assembler.get)
                 nursery.start_soon(lambda: alist(self.assembler.get_iter()))
+        self.assertIsInstance(raised.exception.exceptions[0], ConcurrencyError)
 
     async def test_get_iter_fails_when_get_iter_is_running(self):
         """get_iter cannot be called concurrently."""
-        with trio.testing.RaisesGroup(ConcurrencyError):
+        with self.assertRaises(ExceptionGroup) as raised:
             async with trio.open_nursery() as nursery:
                 nursery.start_soon(lambda: alist(self.assembler.get_iter()))
                 nursery.start_soon(lambda: alist(self.assembler.get_iter()))
+        self.assertIsInstance(raised.exception.exceptions[0], ConcurrencyError)
 
     # Test setting limits.