Stop using configs from the local tor install when we launch tor for tests

We should stop using paths in /usr when we're launching tor for build tests.

We don't get full logs for all the tor processes we launch, but these scripts launch tor, so they're probably affected:

test_rebind.sh
test_key_expiration.sh
test_keygen.sh
test_zero_length_keys.sh

test-network.sh is also affected, but we'll deal with that in chutney in #29701 (moved).

For example, on my machine, test_rebind.sh does:

2019-03-05 11:00:11.573 Tor logged: "Mar 05 11:00:11.571 [notice] Configuration file "/usr/local/etc/tor/torrc" not present, using reasonable defaults.", waiting for "Opened Control listener on"
2019-03-05 11:00:11.596 Tor logged: "Mar 05 11:00:11.591 [notice] Parsing GEOIP IPv4 file /usr/local/share/tor/geoip.", waiting for "Opened Socks listener"
2019-03-05 11:00:11.899 Tor logged: "Mar 05 11:00:11.899 [notice] Parsing GEOIP IPv6 file /usr/local/share/tor/geoip6.", waiting for "Opened Socks listener"

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