refactor: introduce and use InputOrStaticDefault (#632)

This commit introduces a new `InputLoader` policy by which, if no
input is provided, we use a static default input list.

We also modify the code to use this policy for dnscheck and
stunreachability, with proper input.

We also modify `miniooni` to pass the new `ExperimentName` field to
the `InputLoader` to indicate which default input list to use.

This diff is part of a set of diffs aiming at fixing
https://github.com/ooni/probe/issues/1814 and has been
extracted from https://github.com/ooni/probe-cli/pull/539.

What remains to be done, after this diff has landed is to ensure
things also work for ooniprobe and oonimkall.

internal/cmd/miniooni/libminiooni.go

@@ -416,10 +416,11 @@ func MainWithConfiguration(experimentName string, currentOptions Options) {
 			OnWiFi:   true, // meaning: not on 4G
 			Charging: true,
 		},
-		InputPolicy:  builder.InputPolicy(),
-		StaticInputs: currentOptions.Inputs,
-		SourceFiles:  currentOptions.InputFilePaths,
-		Session:      sess,
+		ExperimentName: experimentName,
+		InputPolicy:    builder.InputPolicy(),
+		StaticInputs:   currentOptions.Inputs,
+		SourceFiles:    currentOptions.InputFilePaths,
+		Session:        sess,
 	}
 	inputs, err := inputLoader.Load(context.Background())
 	fatalOnError(err, "cannot load inputs")