refactor(tunnel): provide TorArgs and TorBinary directly (#293)

We're trying to remove a circular dependency between the measurement
Session and the tunnel package. To this end, continue to reduce the
dependency scope by providing TorArgs and TorBinary directly.

Part of https://github.com/ooni/probe/issues/985

internal/engine/tunnel/tunnel.go

@@ -13,8 +13,6 @@ import (
 type Session interface {
 	FetchPsiphonConfig(ctx context.Context) ([]byte, error)
 	TempDir() string
-	TorArgs() []string
-	TorBinary() string
 }
 
 // Tunnel is a tunnel used by the session