Feat: rework Surfshark servers data (#575)

- Feat: `MULTIHOP_ONLY` variable
- Feat: `COUNTRY` variable
- Feat: `CITY` variable
- Feat: `REGION` variable, with retro-compatibility
- Feat: merge servers from API, zip and hardcoded hostnames
- Fix: remove outdated and duplicate servers
- Maint: faster update with fully parallel DNS resolutions

internal/updater/providers/surfshark/sort_test.go

@@ -0,0 +1,42 @@
+package surfshark
+
+import (
+	"testing"
+
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_sortServers(t *testing.T) {
+	t.Parallel()
+	testCases := map[string]struct {
+		initialServers []models.SurfsharkServer
+		sortedServers  []models.SurfsharkServer
+	}{
+		"no server": {},
+		"sorted servers": {
+			initialServers: []models.SurfsharkServer{
+				{Region: "Z", Country: "B", City: "A", Hostname: "A"},
+				{Country: "B", City: "A", Hostname: "A"},
+				{Country: "A", City: "A", Hostname: "B"},
+				{Country: "A", City: "A", Hostname: "A"},
+				{Country: "A", City: "B", Hostname: "A"},
+			},
+			sortedServers: []models.SurfsharkServer{
+				{Country: "A", City: "A", Hostname: "A"},
+				{Country: "A", City: "A", Hostname: "B"},
+				{Country: "A", City: "B", Hostname: "A"},
+				{Country: "B", City: "A", Hostname: "A"},
+				{Region: "Z", Country: "B", City: "A", Hostname: "A"},
+			},
+		},
+	}
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			sortServers(testCase.initialServers)
+			assert.Equal(t, testCase.sortedServers, testCase.initialServers)
+		})
+	}
+}