feature: use go-libtor on mobile, OONI_TOR_BINARY env on desktop (#614)

This branch adds support for running:

1. `go-libtor` on mobile.

2. the tor provided by the desktop app via the `OONI_TOR_BINARY` environment variable.

See https://github.com/ooni/ooni.org/issues/761.

Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/tunnel/tordesktop.go

@@ -0,0 +1,23 @@
+//go:build !android && !ios
+
+package tunnel
+
+// This file implements our strategy for running tor on desktop.
+
+import "github.com/cretz/bine/tor"
+
+// getTorStartConf in this configuration uses torExePath to get a
+// suitable tor binary and then executes it.
+func getTorStartConf(config *Config, dataDir string, extraArgs []string) (*tor.StartConf, error) {
+	exePath, err := config.torBinary()
+	if err != nil {
+		return nil, err
+	}
+	config.logger().Infof("tunnel: tor: exec binary: %s", exePath)
+	return &tor.StartConf{
+		ExePath:   exePath,
+		DataDir:   dataDir,
+		ExtraArgs: extraArgs,
+		NoHush:    true,
+	}, nil
+}