The bug is still here. It looks like a race condition triggered on very slow buildbot like AMD64 FreeBSD 10-STABLE Non-Debug 3.x:

https://buildbot.python.org/all/#/builders/167/builds/728

0:18:13 load avg: 4.84 [405/420/1] test__xxsubinterpreters failed -- running: test_tools (6 min 1 sec)
spam
test_bad_id (test.test__xxsubinterpreters.ChannelIDTests) ... ok
(...)
test_initial (test.test__xxsubinterpreters.ListAllTests) ... ok
test_SystemExit (test.test__xxsubinterpreters.RunStringTests) ... ok
test_already_running (test.test__xxsubinterpreters.RunStringTests) ... FAIL
test_bad_id (test.test__xxsubinterpreters.RunStringTests) ... Exception in thread Thread-9:
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd10.nondebug/build/Lib/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/home/buildbot/python/3.x.koobs-freebsd10.nondebug/build/Lib/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/home/buildbot/python/3.x.koobs-freebsd10.nondebug/build/Lib/test/test__xxsubinterpreters.py", line 51, in run
    interpreters.run_string(interp, dedent(f"""
RuntimeError: interpreter already running

ok
test_bad_script (test.test__xxsubinterpreters.RunStringTests) ... ok
test_bytes_for_script (test.test__xxsubinterpreters.RunStringTests) ... ok
(...)

======================================================================
FAIL: test_already_running (test.test__xxsubinterpreters.RunStringTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd10.nondebug/build/Lib/test/test__xxsubinterpreters.py", line 855, in test_already_running
    interpreters.run_string(self.id, 'print("spam")')
AssertionError: RuntimeError not raised

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

Ran 112 tests in 12.936s

FAILED (failures=1, skipped=5)
test test__xxsubinterpreters failed
(...)
Re-running test 'test__xxsubinterpreters' in verbose mode
(...)
Ran 112 tests in 3.058s
OK (skipped=5)