guardmgr::..::sample_test: Fix intermittent failure.
This test should only fail very rarely (around 1/2.4e8) when guards are chosen from a list of 20 with uniform probability. But that wasn't what we were doing on the mock test network: we were choosing from a list of 10 viable guards, with nonuniform probability. As a fix, we change the test network probabilities so that the guards _are_ chosen with a uniform probability for this test, and we use a modified version of the test network where there are indeed 20 Guard-flagged relays with the required DirCache=2 protocol. Closes #276.