bpo-33735: Fix test_multiprocessing random failure (GH-8059) (GH-8061)

bpo-33735: Fix test_multiprocessing random failure (GH-8059) (GH-8061)

When hunting memory leaks using -R 3:3, test_imap_unordered() of
test_multiprocessing leaks randomly a few memory blocks. It is a
false alarm: when testing using -R 3:20 for example, no leak is
detected.

Modify test_imap_unordered() to be closer to test_imap():

* Only test 10 numbers instead of 1000: it's a pool of 4 processes, so
  10 is enough to test at least one number per process
* Use chunksize=100 instead of chunksize=53 to mimick test_imap()

(cherry picked from commit 23401fb960bb94e6ea62d2999527968d53d3fc65)