feat(acls): rewrite functions to be testable

Rewrite some function to get rid of the dependency on Headscale object. This allows us
to write succinct test that are more easy to review and implement.

The improvements of the tests allowed to write the removal of the tagged hosts
from the namespace as specified here: https://tailscale.com/kb/1068/acl-tags/

machine.go

@@ -119,6 +119,19 @@ func (machine Machine) isExpired() bool {
 	return time.Now().UTC().After(*machine.Expiry)
 }
 
+func (h *Headscale) ListAllMachines() ([]Machine, error) {
+	machines := []Machine{}
+	if err := h.db.Preload("AuthKey").
+		Preload("AuthKey.Namespace").
+		Preload("Namespace").
+		Where("registered").
+		Find(&machines).Error; err != nil {
+		return nil, err
+	}
+
+	return machines, nil
+}
+
 func containsAddresses(inputs []string, addrs MachineAddresses) bool {
 	for _, addr := range addrs.ToStringSlice() {
 		if containsString(inputs, addr) {