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

pkg/oonimkall/internal/tasks/runner.go

@@ -80,6 +80,7 @@ func (r *Runner) newsession(logger *ChanLogger) (*engine.Session, error) {
 		SoftwareName:    r.settings.Options.SoftwareName,
 		SoftwareVersion: r.settings.Options.SoftwareVersion,
 		TempDir:         r.settings.TempDir,
+		TunnelDir:       r.settings.TunnelDir,
 	}
 	if r.settings.Options.ProbeServicesBaseURL != "" {
 		config.AvailableProbeServices = []model.Service{{

pkg/oonimkall/internal/tasks/settings.go

@@ -44,6 +44,11 @@ type Settings struct {
 	// for iOS and does not work for Android.
 	TempDir string `json:"temp_dir"`
 
+	// TunnelDir is the directory where to store persistent state
+	// related to circumvention tunnels. This directory is required
+	// only if you want to use the tunnels. Added since 3.10.0.
+	TunnelDir string `json:"tunnel_dir"`
+
 	// Version indicates the version of this structure.
 	Version int64 `json:"version"`
 }