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/time_test.go

@@ -0,0 +1,22 @@
+package testingx
+
+import (
+	"testing"
+	"time"
+)
+
+func TestTimeDeterministic(t *testing.T) {
+	td := &TimeDeterministic{}
+	t0 := td.Now()
+	if !t0.Equal(td.zeroTime) {
+		t.Fatal("invalid t0 value")
+	}
+	t1 := td.Now()
+	if t1.Sub(t0) != time.Second {
+		t.Fatal("invalid t1 value")
+	}
+	t2 := td.Now()
+	if t2.Sub(t1) != time.Second {
+		t.Fatal("invalid t2 value")
+	}
+}