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/testingx/fakefill_test.go

@@ -0,0 +1,91 @@
+package testingx
+
+import (
+	"testing"
+	"time"
+)
+
+// exampleStructure is an example structure we fill.
+type exampleStructure struct {
+	CategoryCodes string
+	CountryCode   string
+	Enabled       bool
+	MaxResults    int64
+	Now           time.Time
+}
+
+func TestFakeFillWorksWithCustomTime(t *testing.T) {
+	var req *exampleStructure
+	ff := &FakeFiller{
+		Now: func() time.Time {
+			return time.Date(1992, time.January, 24, 17, 53, 0, 0, time.UTC)
+		},
+	}
+	ff.Fill(&req)
+	if req == nil {
+		t.Fatal("we expected non nil here")
+	}
+}
+
+func TestFakeFillAllocatesIntoAPointerToPointer(t *testing.T) {
+	var req *exampleStructure
+	ff := &FakeFiller{}
+	ff.Fill(&req)
+	if req == nil {
+		t.Fatal("we expected non nil here")
+	}
+}
+
+func TestFakeFillAllocatesIntoAMapLikeWithStringKeys(t *testing.T) {
+	var resp map[string]*exampleStructure
+	ff := &FakeFiller{}
+	ff.Fill(&resp)
+	if resp == nil {
+		t.Fatal("we expected non nil here")
+	}
+	if len(resp) < 1 {
+		t.Fatal("we expected some data here")
+	}
+	for _, value := range resp {
+		if value == nil {
+			t.Fatal("expected non-nil here")
+		}
+	}
+}
+
+func TestFakeFillAllocatesIntoAMapLikeWithNonStringKeys(t *testing.T) {
+	var panicmsg string
+	func() {
+		defer func() {
+			if v := recover(); v != nil {
+				panicmsg = v.(string)
+			}
+		}()
+		var resp map[int64]*exampleStructure
+		ff := &FakeFiller{}
+		ff.Fill(&resp)
+		if resp != nil {
+			t.Fatal("we expected nil here")
+		}
+	}()
+	if panicmsg != "fakefill: we only support string key types" {
+		t.Fatal("unexpected panic message", panicmsg)
+	}
+}
+
+func TestFakeFillAllocatesIntoASlice(t *testing.T) {
+	var resp *[]*exampleStructure
+	ff := &FakeFiller{}
+	ff.Fill(&resp)
+	if resp == nil {
+		t.Fatal("we expected non nil here")
+	}
+	if len(*resp) < 1 {
+		t.Fatal("we expected some data here")
+	}
+	for _, entry := range *resp {
+		if entry == nil {
+			t.Fatal("expected non-nil here")
+		}
+	}
+}