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.
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