Fix unit test in test_entrynodes.c
The last check in entrynodes/select_for_circuit_no_confirmed
was trivially succeeding because the call to select_entry_guard_for_circuit
was returning NULL
, due to EnforceDistinctSubnets
not being set to 0
. See https://gitlab.torproject.org/tpo/core/tor/-/blob/a56350abc8a8c18bfe0b60c751cce5f851db0d3e/src/test/test_entrynodes.c#L178