Reduce repeats in unpark_all_one_fast test

[Darksonn]

This test is causing failures in Android CI because it is too slow. I suggest reducing the number of repeats.

From our CI:

stdout: 
running 12 tests
test parking_lot::tests::unpark_one_one ... ok <0.499s>
test parking_lot::tests::hundred_unpark_all_one ... ok <0.568s>
test parking_lot::tests::unpark_one_fifty ... ok <0.706s>
test parking_lot::tests::unpark_one_fifty_then_fifty_all ... ok <1.489s>
test parking_lot::tests::unpark_one_hundred_fast ... ok <3.500s>
test parking_lot::tests::unpark_all_one ... ok <6.221s>
test parking_lot::tests::unpark_one_fifty_then_fifty_all_fast ... ok <10.103s>
test parking_lot::tests::unpark_all_hundred_fast ... ok <15.896s>
test parking_lot::tests::unpark_all_hundred ... ok <15.980s>
test parking_lot::tests::hundred_unpark_all_one_fast ... ok <20.817s>
test parking_lot::tests::unpark_one_one_fast ... ok <27.496s>
test parking_lot::tests::unpark_all_one_fast has been running for over 60 seconds

stderr: 
  Test run incomplete. Started 12 tests, finished 11