feat(oonirun): improve tests (#915)

See https://github.com/ooni/probe/issues/2184

While there, rename `runtimex.PanicIfFalse` to `runtimex.Assert` (it was about time...)

internal/model/mocks/saver.go

@@ -0,0 +1,12 @@
+package mocks
+
+import "github.com/ooni/probe-cli/v3/internal/model"
+
+// Saver saves a measurement on some persistent storage.
+type Saver struct {
+	MockSaveMeasurement func(m *model.Measurement) error
+}
+
+func (s *Saver) SaveMeasurement(m *model.Measurement) error {
+	return s.MockSaveMeasurement(m)
+}