[testing] Fix parallel test tear down.

[brendandahl]

Bad merge...

[juj]

Great, thanks!

[juj]

Unfortunately still busted, I'm afraid:

http://clbri.com:8010/api/v2/logs/46836/raw_inline

Traceback (most recent call last):
  File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/runner.py", line 603, in <module>
    sys.exit(main())
             ~~~~^^
  File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/runner.py", line 592, in main
    num_failures = run_tests(options, suites)
  File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/runner.py", line 434, in run_tests
    res = testRunner.run(suite)
  File "/home/clb/.pyenv/versions/3.13.3/lib/python3.13/unittest/runner.py", line 240, in run
    test(result)
    ~~~~^^^^^^^^
  File "/home/clb/.pyenv/versions/3.13.3/lib/python3.13/unittest/suite.py", line 84, in __call__
    return self.run(*args, **kwds)
           ~~~~~~~~^^^^^^^^^^^^^^^
  File "/home/clb/buildbot/h12dsi-linux-mint22/emscripten_linux_x64/build/emscripten/main/test/parallel_testsuite.py", line 139, in run
    assert(num_tear_downs == use_cores)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError
program finished with exit code 1