This issue is not solved. I can still reproduce the hang using:

./python -m test test_asyncio -m SubprocessMultiLoopWatcherTests -v -F -j20 --timeout=30.0

Example with test_cancel_make_subprocess_transport_exec:

$ ./python -m test test_asyncio -m test.test_asyncio.test_subprocess.SubprocessMultiLoopWatcherTests.test_cancel_make_subprocess_transport_exec -v -F -j20 --timeout=30.0
...
0:00:37 load avg: 10.97 [163] test_asyncio passed -- running: test_asyncio (30.9 sec)
test_cancel_make_subprocess_transport_exec (test.test_asyncio.test_subprocess.SubprocessMultiLoopWatcherTests) ... ok

----------------------------------------------------------------------

Ran 1 test in 0.032s

OK
0:00:37 load avg: 10.97 [164] test_asyncio passed -- running: test_asyncio (31.0 sec)
test_cancel_make_subprocess_transport_exec (test.test_asyncio.test_subprocess.SubprocessMultiLoopWatcherTests) ... ok

----------------------------------------------------------------------

Ran 1 test in 0.036s

OK
0:00:37 load avg: 10.97 [165/1] test_asyncio crashed (Exit code 1)
test_cancel_make_subprocess_transport_exec (test.test_asyncio.test_subprocess.SubprocessMultiLoopWatcherTests) ...
Timeout (0:00:30)!
Thread 0x00007fb583cbd740 (most recent call first):
  File "/home/vstinner/python/main/Lib/selectors.py", line 469 in select
  File "/home/vstinner/python/main/Lib/asyncio/base_events.py", line 1845 in _run_once
  File "/home/vstinner/python/main/Lib/asyncio/base_events.py", line 595 in run_forever
  File "/home/vstinner/python/main/Lib/asyncio/base_events.py", line 628 in run_until_complete
  File "/home/vstinner/python/main/Lib/test/test_asyncio/test_subprocess.py", line 442 in test_cancel_make_subprocess_transport_exec
...


Test complete in less than 1 second, but sometimes it hangs for at least 30 seconds.