fix(tunnel): pass /absolute/path/to/tor to cretz/bine (#323)

* fix(tunnel): pass /absolute/path/to/tor to cretz/bine

It seems cretz/bine is not aware of https://blog.golang.org/path-security
for now. I am planning to send over a diff for that later today.

In the meanwhile, do the right thing here, and make sure that we obtain
the absolute path to the tor binary before we continue.

This work is part of https://github.com/ooni/probe-engine/issues/283.

* fix tests when tor is not installed

internal/engine/session.go

@@ -171,6 +171,7 @@ func NewSession(ctx context.Context, config SessionConfig) (*Session, error) {
 			config.Logger.Infof(
 				"starting '%s' tunnel; please be patient...", proxyURL.Scheme)
 			tunnel, err := tunnel.Start(ctx, &tunnel.Config{
+				Logger:    config.Logger,
 				Name:      proxyURL.Scheme,
 				Session:   &sessionTunnelEarlySession{},
 				TorArgs:   config.TorArgs,