feat: dnsping using step-by-step (#831)

Reference issue for this pull request: https://github.com/ooni/probe/issues/2159

This diff refactors the `dnsping` experiment to use the [step-by-step measurement style](https://github.com/ooni/probe-cli/blob/master/docs/design/dd-003-step-by-step.md).

Co-authored-by: decfox <decfox@github.com>
Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/engine/experiment/dnsping/testkeys.go

@@ -0,0 +1,35 @@
+package dnsping
+
+import (
+	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// TestKeys contains the experiment results.
+type TestKeys struct {
+	Pings []*SinglePing `json:"pings"`
+
+	// mu provides mutual exclusion
+	mu sync.Mutex
+}
+
+// SinglePing contains the results of a single ping.
+type SinglePing struct {
+	Query *model.ArchivalDNSLookupResult `json:"query"`
+}
+
+// NewTestKeys creates new dnsping TestKeys
+func NewTestKeys() *TestKeys {
+	return &TestKeys{
+		Pings: []*SinglePing{},
+		mu:    sync.Mutex{},
+	}
+}
+
+// addSinglePing adds []*SinglePing to the test keys
+func (tk *TestKeys) addPings(pings []*SinglePing) {
+	tk.mu.Lock()
+	tk.Pings = append(tk.Pings, pings...)
+	tk.mu.Unlock()
+}