fix(ooniprobe): dnscheck,stunreachability run w/ default input (#633)

This diff is part of https://github.com/ooni/probe/issues/1814 and
teaches `ooniprobe` to run dnscheck and stunreachability by using the
default static input feature of the `InputLoader`.

I've manually tested that we can still run `websites` like
we did before (including category filtering).

I've also manually tested that now we can run `experimental` and
get parseable results for dnscheck and stunreachability.

With this diff in, we have fixed the original problem highlighted in
the https://github.com/ooni/probe/issues/1814 issue.

Yet, because of the way in which I solved the problem, there is
more work to do. My changes have broken stunreachability for
mobile and now it's time I apply fixes to make it work again.

This diff was extracted from https://github.com/ooni/probe-cli/pull/539,
which at this point only basically contains the remaining fixes to
ensure we can run stunreachability on mobile.

Co-authored-by: Arturo Filastò <arturo@filasto.net>

Co-authored-by: Arturo Filastò <arturo@filasto.net>

cmd/ooniprobe/internal/nettests/stunreachability.go

@@ -1,13 +1,42 @@
 package nettests
 
+import (
+	"context"
+
+	engine "github.com/ooni/probe-cli/v3/internal/engine"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
 // STUNReachability nettest implementation.
 type STUNReachability struct{}
 
+func (n STUNReachability) lookupURLs(ctl *Controller) ([]string, error) {
+	inputloader := &engine.InputLoader{
+		CheckInConfig: &model.CheckInConfig{
+			// not needed because we have default static input in the engine
+		},
+		ExperimentName: "stunreachability",
+		InputPolicy:    engine.InputOrStaticDefault,
+		Session:        ctl.Session,
+		SourceFiles:    ctl.InputFiles,
+		StaticInputs:   ctl.Inputs,
+	}
+	testlist, err := inputloader.Load(context.Background())
+	if err != nil {
+		return nil, err
+	}
+	return ctl.BuildAndSetInputIdxMap(ctl.Probe.DB(), testlist)
+}
+
 // Run starts the nettest.
 func (n STUNReachability) Run(ctl *Controller) error {
 	builder, err := ctl.Session.NewExperimentBuilder("stunreachability")
 	if err != nil {
 		return err
 	}
-	return ctl.Run(builder, []string{""})
+	urls, err := n.lookupURLs(ctl)
+	if err != nil {
+		return err
+	}
+	return ctl.Run(builder, urls)
 }