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/engine/saver.go

@@ -6,10 +6,8 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
-// Saver saves a measurement on some persistent storage.
-type Saver interface {
-	SaveMeasurement(m *model.Measurement) error
-}
+// Saver is an alias for model.Saver.
+type Saver = model.Saver
 
 // SaverConfig is the configuration for creating a new Saver.
 type SaverConfig struct {