Pick random dashboard port in tests (dask#8965)

distributed/tests/test_preload.py

@@ -238,7 +238,7 @@ async def test_web_preload_worker():
         "urllib3.PoolManager.request",
         **{"return_value.data": data},
     ) as request:
-        async with Scheduler(port=port, host="localhost") as s:
+        async with Scheduler(port=port, host="localhost", dashboard_address=":0") as s:
             async with Nanny(preload_nanny=["http://example.com/preload"]) as nanny:
                 assert nanny.scheduler_addr == s.address
     assert request.mock_calls == [

distributed/tests/test_scheduler.py

@@ -4523,7 +4523,7 @@ def test_runspec_regression_sync(loop):
     # https://github.com/dask/distributed/issues/6624
     np = pytest.importorskip("numpy")
     da = pytest.importorskip("dask.array")
-    with Client(loop=loop):
+    with Client(loop=loop, dashboard_address=":0"):
         v = da.random.random((20, 20), chunks=(5, 5))
 
         overlapped = da.map_overlap(np.sum, v, depth=2, boundary="reflect")

distributed/tests/test_utils_test.py

@@ -1008,9 +1008,9 @@ def test_sizeof_error(input, exc, msg):
 @gen_test()
 async def test_ensure_no_new_clients():
     with ensure_no_new_clients():
-        async with Scheduler() as s:
+        async with Scheduler(dashboard_address=":0") as s:
             pass
-    async with Scheduler() as s:
+    async with Scheduler(dashboard_address=":0") as s:
         with ensure_no_new_clients():
             pass
         with pytest.raises(AssertionError):

distributed/tests/test_worker.py

@@ -505,7 +505,7 @@ def setup(self, worker=None):
 
 @gen_test()
 async def test_plugin_internal_exception():
-    async with Scheduler(port=0) as s:
+    async with Scheduler(port=0, dashboard_address=":0") as s:
         with raises_with_cause(
             RuntimeError,
             "Worker failed to start",