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/httpx/httpx_test.go

@@ -13,10 +13,10 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
-	"github.com/ooni/probe-cli/v3/internal/fakefill"
 	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/model/mocks"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/testingx"
 	"github.com/ooni/probe-cli/v3/internal/version"
 )
 
@@ -47,7 +47,7 @@ func TestAPIClientTemplate(t *testing.T) {
 			HTTPClient: http.DefaultClient,
 			Logger:     model.DiscardLogger,
 		}
-		ff := &fakefill.Filler{}
+		ff := &testingx.FakeFiller{}
 		ff.Fill(tmpl)
 		ac := tmpl.Build()
 		orig := apiClient(*tmpl)
@@ -64,7 +64,7 @@ func TestAPIClientTemplate(t *testing.T) {
 			HTTPClient: http.DefaultClient,
 			Logger:     model.DiscardLogger,
 		}
-		ff := &fakefill.Filler{}
+		ff := &testingx.FakeFiller{}
 		ff.Fill(tmpl)
 		tok := ""
 		ff.Fill(&tok)
@@ -188,7 +188,7 @@ func TestAPIClient(t *testing.T) {
 
 		t.Run("sets the content-type properly", func(t *testing.T) {
 			var jsonReq fakeRequest
-			ff := &fakefill.Filler{}
+			ff := &testingx.FakeFiller{}
 			ff.Fill(&jsonReq)
 			client := newAPIClient()
 			req, err := client.newRequestWithJSONBody(