refactor(tracex): convert to unit testing (#781)

The exercise already allowed me to notice issues such as fields not
being properly initialized by savers.

This is one of the last steps before moving tracex away from the
internal/netx package and into the internal package.

See https://github.com/ooni/probe/issues/2121

internal/engine/netx/tracex/saver_test.go

@@ -3,22 +3,88 @@ package tracex
 import (
 	"sync"
 	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model/mocks"
 )
 
 func TestSaver(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("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")
+			}
+		})
+	})
 }