Add missing long time tag to a failing doctest

[kwankyu]

To fix

sage -t --warn-long 5.0 --random-seed=279311403945742486210819617974441498089 src/sage/parallel/map_reduce.py
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File "src/sage/parallel/map_reduce.py", line 1145, in sage.parallel.map_reduce.RESetMapReduce.start_workers
Failed example:
    S.finish()
Exception raised:
    Traceback (most recent call last):
      File "/Users/kwankyu/GitHub/sage-dev/src/sage/doctest/forker.py", line 715, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/Users/kwankyu/GitHub/sage-dev/src/sage/doctest/forker.py", line 1136, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.parallel.map_reduce.RESetMapReduce.start_workers[4]>", line 1, in <module>
        S.finish()
      File "/Users/kwankyu/GitHub/sage-dev/src/sage/parallel/map_reduce.py", line 1239, in finish
        worker.join()
      File "/usr/local/Cellar/[email protected]/3.12.7/Frameworks/Python.framework/Versions/3.12/lib/python3.12/multiprocessing/process.py", line 148, in join
        assert self._popen is not None, 'can only join a started process'
               ^^^^^^^^^^^^^^^^^^^^^^^
    AssertionError: can only join a started process
**********************************************************************

📝 Checklist

• The title is concise and informative.
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• I have created tests covering the changes.
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⌛ Dependencies

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[tornaria]

Duplicate of #38873 ?

[kwankyu]

A bit different solution for the same problem. I think the fix of this PR is more to the point.

Reviewer may choose a better one.

[kwankyu]

Ah, it already got positive review. OK. I will close this one.