refactor: move tracex outside of engine/netx (#782)

* refactor: move tracex outside of engine/netx

Consistently with https://github.com/ooni/probe/issues/2121 and
https://github.com/ooni/probe/issues/2115, we can now move tracex
outside of engine/netx. The main reason why this makes sense now
is that the package is now changed significantly from the one
that we imported from ooni/probe-engine.

We have improved its implementation, which had not been touched
significantly for quite some time, and converted it to unit
testing. I will document tomorrow some extra work I'd like to
do with this package but likely could not do $soon.

* go fmt

* regen tutorials

internal/tracex/saver_test.go

@@ -0,0 +1,90 @@
+package tracex
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model/mocks"
+)
+
+func TestSaver(t *testing.T) {
+	t.Run("concurrent writes followed by read", func(t *testing.T) {
+		saver := Saver{}
+		var wg sync.WaitGroup
+		const parallel = 10
+		wg.Add(parallel)
+		for idx := 0; idx < parallel; idx++ {
+			go func() {
+				saver.Write(&EventReadFromOperation{&EventValue{}})
+				wg.Done()
+			}()
+		}
+		wg.Wait()
+		ev := saver.Read()
+		if len(ev) != parallel {
+			t.Fatal("unexpected number of events read")
+		}
+	})
+
+	t.Run("NewConnectObserver", func(t *testing.T) {
+		t.Run("nil Saver", func(t *testing.T) {
+			var saver *Saver
+			obs := saver.NewConnectObserver()
+			if obs != nil {
+				t.Fatal("expected nil observer")
+			}
+		})
+
+		t.Run("nonnnil Saver", func(t *testing.T) {
+			saver := &Saver{}
+			obs := saver.NewConnectObserver()
+			underlying := obs.(*dialerConnectObserver)
+			if underlying.saver != saver {
+				t.Fatal("invalid saver")
+			}
+		})
+	})
+
+	t.Run("NewReadWriteObserver", func(t *testing.T) {
+		t.Run("nil Saver", func(t *testing.T) {
+			var saver *Saver
+			obs := saver.NewReadWriteObserver()
+			if obs != nil {
+				t.Fatal("expected nil observer")
+			}
+		})
+
+		t.Run("nonnnil Saver", func(t *testing.T) {
+			saver := &Saver{}
+			obs := saver.NewReadWriteObserver()
+			underlying := obs.(*dialerReadWriteObserver)
+			if underlying.saver != saver {
+				t.Fatal("invalid saver")
+			}
+		})
+	})
+
+	t.Run("WrapQUICDialer", func(t *testing.T) {
+		t.Run("nil Saver", func(t *testing.T) {
+			var saver *Saver
+			base := &mocks.QUICDialer{}
+			qd := saver.WrapQUICDialer(base)
+			if qd != base {
+				t.Fatal("unexpected returned QUICDialer")
+			}
+		})
+
+		t.Run("nonnnil Saver", func(t *testing.T) {
+			saver := &Saver{}
+			base := &mocks.QUICDialer{}
+			qd := saver.WrapQUICDialer(base)
+			underlying := qd.(*QUICDialerSaver)
+			if underlying.Saver != saver {
+				t.Fatal("invalid Saver")
+			}
+			if underlying.QUICDialer != base {
+				t.Fatal("invalid QUICDialer")
+			}
+		})
+	})
+}