test_embed: test_thread_state_ensure() failed on "AMD64 Debian root 3.15" buildbot

AMD64 Debian root 3.15: https://buildbot.python.org/#/builders/2019/builds/85

FAIL: test_thread_state_ensure (test.test_embed.AuditingTests.test_thread_state_ensure)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/test_embed.py", line 2000, in test_thread_state_ensure
    self.run_embedded_interpreter("test_thread_state_ensure")
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/test_embed.py", line 138, in run_embedded_interpreter
    self.assertEqual(p.returncode, returncode,
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
                     "bad returncode %d, stderr is %r" %
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                     (p.returncode, err))
                     ^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 0 : bad returncode -6, stderr is "_testembed: ./Programs/_testembed.c:2736: test_thread_state_ensure: Assertion `_Py_atomic_load_int(&data.done) == 1' failed.\n"

Stdout:
--- ['/root/buildarea/3.15.angelico-debian-amd64/build/Programs/_testembed', 'test_thread_state_ensure'] failed ---
stdout:

stderr:
_testembed: ./Programs/_testembed.c:2736: test_thread_state_ensure: Assertion `_Py_atomic_load_int(&data.done) == 1' failed.

cc @ZeroIntensity

从 Lib/test/test_embed.py 开始，尤其检查 AuditingTests.test_thread_state_ensure 和 run_embedded_interpreter，然后检查 Programs/_testembed.c 中 test_thread_state_ensure 里失败断言附近的代码。在 AMD64 Debian buildbot 配置上重现该失败，并确定需要什么才能使测试在 data.done 等于 1 的情况下完成并返回代码 0。