Pytest cannot be run without forked

[Gamenot]

BUG REPORT

High Level Description
By the problems that ULTRA team were having, pytest has stability issues running tests without forked. Tests with forked take a much longer time to run. This is due to issues with having multiple SMARTS instances.

SMARTS version
SMARTS branch: ultra-gb-record-density-data
2611ccc

Previous associated issues
#719
#597
#184

Steps to reproduce the bug
Run the CI on the 2611ccc commit
Use branch listed here: #745

Resulting and expected behaviour
:display:x11display(fatal) thrown when all tests should pass.

Error logs and screenshots
#730
https://github.com/huawei-noah/SMARTS/runs/2279454801

System information

• Operating system & Environment [Please note if using Docker]:
  Github CI: Linux 18.04 Docker
• Number of system cores available
  2
• Python version [Please note if using Jupyter]
  3.7
• Python packages [e.g. from pip freeze]
  n/a
• SUMO version
  n/a
• Date when bug occurred
  2021/04/07

Impact [If known]
CI is much slower due to having to run forked.

[sah-huawei]

With PR #747, I was just able to run make test without --forked on my local laptop (although I had to hack -n 3 to prevent overload!). All tests passed.

@Gamenot do you think we should remove --forked from the test rule in the Makefile? Or just do that for CI? (The -n issue is still annoying when running locally.)

EDIT: Never mind. I get seemingly-random failures if I play with -n or test ordering. I guess there's still more digging to do!

[sah-huawei]

I suspect the problems may arise when running two tests that use Ray side-by-side. For example, (after reordering the tests), I got a segmentation fault (in test_multi_instance_example()) when test_rllib_hiway_env() (from ./env/tests/test_rllib_hiway_env.py) was being executed at the same time as test_multi_instance_example() (from ./tests/tests_examples.py), both of which have calls to ray.init().