Maint: dns package interface rework

- return concrete struct type
- split interface is sub-interfaces

internal/dns/run.go

@@ -7,7 +7,11 @@ import (
 	"github.com/qdm12/gluetun/internal/constants"
 )
 
-func (l *looper) Run(ctx context.Context, done chan<- struct{}) {
+type Runner interface {
+	Run(ctx context.Context, done chan<- struct{})
+}
+
+func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 	defer close(done)
 
 	const fallback = false