feat(tunnel): introduce persistent tunnel state dir (#294)

* feat(tunnel): introduce persistent tunnel state dir

This diff introduces a persistent state directory for tunnels, so that
we can bootstrap them more quickly after the first time.

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

* fix: make tunnel dir optional

We have many tests where it does not make sense to explicitly
provide a tunnel dir because we're not using tunnels.

This should simplify setting up a session.

* fix(tunnel): repair tests

* final changes

* more cleanups

cmd/ooniprobe/internal/utils/paths.go

@@ -30,6 +30,11 @@ func AssetsDir(home string) string {
 	return filepath.Join(home, "assets")
 }
 
+// TunnelDir returns the directory where to store tunnels state
+func TunnelDir(home string) string {
+	return filepath.Join(home, "tunnel")
+}
+
 // EngineDir returns the directory where ooni/probe-engine should
 // store its private data given a specific OONI Home.
 func EngineDir(home string) string {