This test, in test_client_pick_intro(), will have different coverage
depending on whether it selects a good intro point the first time or
whether it has to try a few times. Since it produces the shorter
coverage with P=1/4, repeat this test 64 times so that it only
provides reduced coverage with P=1/2^128. The performance cost is
negligible.
Closes ticket 25996. This test was introduced in 0.3.2.1-alpha.
