Faster servers information updater (#248)

* Asynchronous repeatResolve
* Parallel cyberghost and PIA (v3) processing, with a 10 goroutines limit
* Add missing vyprvpn cli flag to updater
* Increase DNS repetitions to 5 in order to obtain more IP addresses
* Update old PIA IP addresses
* Add Surfshark servers by API (unused for now)

internal/updater/resolver_test.go

@@ -0,0 +1,75 @@
+package updater
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_resolveRepeat(t *testing.T) {
+	t.Parallel()
+	testCases := map[string]struct {
+		lookupIPResult [][]net.IP
+		lookupIPErr    error
+		n              int
+		ips            []net.IP
+		err            error
+	}{
+		"failure": {
+			lookupIPResult: [][]net.IP{
+				{{1, 1, 1, 1}, {1, 1, 1, 2}},
+			},
+			lookupIPErr: fmt.Errorf("feeling sick"),
+			n:           1,
+			ips:         []net.IP{},
+			err:         fmt.Errorf("feeling sick"),
+		},
+		"successful": {
+			lookupIPResult: [][]net.IP{
+				{{1, 1, 1, 1}, {1, 1, 1, 2}},
+				{{2, 1, 1, 1}, {2, 1, 1, 2}},
+				{{2, 1, 1, 3}, {2, 1, 1, 2}},
+			},
+			n: 3,
+			ips: []net.IP{
+				{1, 1, 1, 1},
+				{1, 1, 1, 2},
+				{2, 1, 1, 1},
+				{2, 1, 1, 2},
+				{2, 1, 1, 3},
+			},
+		},
+	}
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			if testCase.lookupIPErr == nil {
+				require.Len(t, testCase.lookupIPResult, testCase.n)
+			}
+			const host = "blabla"
+			i := 0
+			lookupIP := func(ctx context.Context, argHost string) (
+				ips []net.IP, err error) {
+				assert.Equal(t, host, argHost)
+				result := testCase.lookupIPResult[i]
+				i++
+				return result, testCase.err
+			}
+
+			ips, err := resolveRepeat(
+				context.Background(), lookupIP, host, testCase.n)
+			if testCase.err != nil {
+				require.Error(t, err)
+				assert.Equal(t, testCase.err.Error(), err.Error())
+			} else {
+				assert.NoError(t, err)
+			}
+			assert.Equal(t, testCase.ips, ips)
+		})
+	}
+}