[3.13] gh-124402: Speed up test_free_threading and test_super (#124491)

[vstinner]

• Reduce the number of iterations and the number of threads so a whole test file takes less than a minute.
• Refactor test_racing_iter_extend() to remove two levels of indentation.
• test_monitoring() uses a sleep of 100 ms instead of 1 second.

(cherry picked from commit 0387c34)

• Issue: test_free_threading and test_super take way to long on a Free Threaded build. #124402