bpo-40275: Fix a calling bug in threading_helper

[shihai1991]

https://bugs.python.org/issue40275

[shihai1991]

TODO: test tools should have unittest too.

[shihai1991]

@vstinner Hi, victor. Pls take a look if you have free time. Sorry, I introduced a potential bug in threading_helper :(

[vstinner]

Oh right, there are a few more:

$ pyflakes Lib/test/support/threading_helper.py Lib/test/support/threading_helper.py:50:9 undefined name 'gc_collect'
Lib/test/support/threading_helper.py:101:13 undefined name 'gc_collect'
Lib/test/support/threading_helper.py:127:16 undefined name 'verbose'
Lib/test/support/threading_helper.py:136:23 local variable 'starttime' is assigned to but never used
Lib/test/support/threading_helper.py:144:20 undefined name 'verbose'
Lib/test/support/threading_helper.py:150:45 undefined name 'sys'

[vstinner]

These bugs caused multiple buildbots failures: #20448 (comment)

I created PR #20488 to fix all pyflakes warning.

[vstinner]

@vstinner Hi, victor. Pls take a look if you have free time. Sorry, I introduced a potential bug in threading_helper :(

That's ok. I didn't spot these issues neither, and test_support passed.

[vstinner]

I merged my #20488 instead.

[shihai1991]

I merged my #20488 instead.

Got it, thanks.