refactor: move more commands to internal/cmd (#207)

* refactor: move more commands to internal/cmd

Part of https://github.com/ooni/probe/issues/1335.

We would like all commands to be at the same level of engine
rather than inside engine (now that we can do it).

* fix: update .gitignore

* refactor: also move jafar outside engine

* We should be good now?

internal/cmd/oohelperd/oohelperd_test.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestSmoke(t *testing.T) {
+	// Just check whether we can start and then tear down the server, so
+	// we have coverage of this code and when we see that some lines aren't
+	// covered we know these are genuine places where we're not testing
+	// the code rather than just places like this simple main.
+	go testableMain()
+	srvcancel()  // kills the listener
+	srvwg.Wait() // joined
+}