feat: tlsping and tcpping using step-by-step (#815)

## Checklist

- [x] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: https://github.com/ooni/probe/issues/2158
- [x] if you changed anything related how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request: https://github.com/ooni/spec/pull/250

## Description

This diff refactors the codebase to reimplement tlsping and tcpping
to use the step-by-step measurements style.

See docs/design/dd-003-step-by-step.md for more information on the
step-by-step measurement style.

internal/netxlite/trace_test.go

@@ -0,0 +1,40 @@
+package netxlite
+
+import (
+	"context"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model/mocks"
+)
+
+func TestContextTraceOrDefault(t *testing.T) {
+	t.Run("without a configured trace we get a default", func(t *testing.T) {
+		ctx := context.Background()
+		tx := ContextTraceOrDefault(ctx)
+		_ = tx.(*traceDefault) // panic if cannot cast
+	})
+
+	t.Run("with a configured trace we get the expected trace", func(t *testing.T) {
+		realTrace := &mocks.Trace{}
+		ctx := ContextWithTrace(context.Background(), realTrace)
+		tx := ContextTraceOrDefault(ctx)
+		if tx != realTrace {
+			t.Fatal("not the trace we expected")
+		}
+	})
+}
+
+func TestContextWithTrace(t *testing.T) {
+	t.Run("panics if passed a nil trace", func(t *testing.T) {
+		var called bool
+		func() {
+			defer func() {
+				called = (recover() != nil)
+			}()
+			_ = ContextWithTrace(context.Background(), nil)
+		}()
+		if !called {
+			t.Fatal("not called")
+		}
+	})
+}