$ ./python -m test -j2 test__xxsubinterpreters test__xxsubinterpreters -m test_already_running -v
(...)
== Python build: debug TraceRefs
(...)
== CPU count: 12
(...)
0:00:00 load avg: 1.19 Run tests in parallel using 2 child processes
0:00:00 load avg: 1.19 [1/2] test__xxsubinterpreters passed
test_already_running (test.test__xxsubinterpreters.RunStringTests.test_already_running) ... ok
(...)

0:00:00 load avg: 1.19 [2/2/1] test__xxsubinterpreters crashed (Exit code -6)
test_already_running (test.test__xxsubinterpreters.RunStringTests.test_already_running) ...

Objects/object.c:2235: _Py_ForgetReference: Assertion failed: invalid object chain
Enable tracemalloc to get the memory block allocation traceback

object address  : 0x7f01d0be95a0
object refcount : 0
object type     : 0xa1dc00
object type name: str
object repr     : <refcnt 0 at 0x7f01d0be95a0>

Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: initialized

Current thread 0x00007f01df4a1740 (most recent call first):
  <no Python frame>
(...)

git clean -fdx
./configure --with-pydebug --with-trace-refs CFLAGS="-O0"
make -j14