Make gen_tests show up in VSCode test discovery (#6424)

Not entirely sure why, but `@functools.wraps` on the test function makes them show up. This is convenient when VSCode users want to debug a `@gen_test` test.

distributed/utils_test.py

@@ -816,6 +816,7 @@ async def test_foo():
         timeout = 3600
 
     def _(func):
+        @functools.wraps(func)
         def test_func(*args, **kwargs):
             with clean(**clean_kwargs) as loop:
                 injected_func = functools.partial(func, *args, **kwargs)