Local test failures on MacOS

I can't seem to create a working environment on my MacOS Catalina dev box. CONTRIBUTING.md could use some attention...

```
export PYTHON=3.7 TESTS=true PACKAGES="scikit-learn python-snappy python-blosc" TORNADO=6
source continuous_integration/travis/install.sh
source continuous_integration/travis/setup-ssh.sh
source continuous_integration/travis/run_tests.sh
```
Test failures:

# 1.
Many (all?) tests failed on the ``check_thread_leak()`` helper function, e.g. test_scheduler_bokeh.test_simple:
```
> assert False, (thread, call_stacks)
E AssertionError: (, [' File "/Users/crusaderky/miniconda3/envs/distributed/lib/python...ile "/Users/crusaderky/PycharmProjects/distributed/distributed/profile.py", line 269, in _watch
E \tsleep(interval)
E '])
E assert False
```

# 2.
- test_dask_scheduler.test_defaults
- test_dask_scheduler.test_hostport
- test_comms.test_default_client_server_ipv6
- test_comms.test_tcp_client_server_ipv4
- test_comms.test_tcp_client_server_ipv6
- test_comms.test_tls_client_server_ipv4

failed with:
OSError: Timed out trying to connect to 'tcp://[Guidos-MacBook-Pro.local]:8786' after 5 s: Timed out trying to connect to 'tcp://[Guidos-MacBook-Pro.local]:8786' after 5 s: connect() didn't finish in time

# 3.
- test_dask_scheduler.test_no_dashboard
- test_dask_scheduler.test_dashboard

failed with:
```
with pytest.raises(Exception):
> requests.get("http://127.0.0.1:8787/status/")
E Failed: DID NOT RAISE
```

What am I missing or doing wrong? Have the tests been run recently on MacOSX?