IVPN server data update code and ISP filter (#578)

- Use IVPN's HTTP API instead of their .zip file
- Unit tests for API and GetServers
- Paves the way for Wireguard
- Update server information for IVPN
- Add `ISP` filter for IVPN

internal/updater/providers/ivpn/api_test.go

@@ -0,0 +1,96 @@
+package ivpn
+
+import (
+	"context"
+	"errors"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_fetchAPI(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		responseStatus int
+		responseBody   io.ReadCloser
+		data           apiData
+		err            error
+	}{
+		"http response status not ok": {
+			responseStatus: http.StatusNoContent,
+			err:            errors.New("HTTP status code not OK: 204 No Content"),
+		},
+		"nil body": {
+			responseStatus: http.StatusOK,
+			err:            errors.New("failed unmarshaling response body: EOF"),
+		},
+		"no server": {
+			responseStatus: http.StatusOK,
+			responseBody:   ioutil.NopCloser(strings.NewReader(`{}`)),
+		},
+		"success": {
+			responseStatus: http.StatusOK,
+			responseBody: ioutil.NopCloser(strings.NewReader(`{"servers":[
+				{"country":"Country1","city":"City A","isp":"xyz","is_active":true,"hostnames":{"openvpn":"hosta"}},
+				{"country":"Country2","city":"City B","isp":"abc","is_active":false,"hostnames":{"openvpn":"hostb"}}
+			]}`)),
+			data: apiData{
+				Servers: []apiServer{
+					{
+						Country:  "Country1",
+						City:     "City A",
+						IsActive: true,
+						ISP:      "xyz",
+						Hostnames: apiHostnames{
+							OpenVPN: "hosta",
+						},
+					},
+					{
+						Country: "Country2",
+						City:    "City B",
+						ISP:     "abc",
+						Hostnames: apiHostnames{
+							OpenVPN: "hostb",
+						},
+					},
+				},
+			},
+		},
+	}
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx := context.Background()
+
+			client := &http.Client{
+				Transport: roundTripFunc(func(r *http.Request) (*http.Response, error) {
+					assert.Equal(t, http.MethodGet, r.Method)
+					assert.Equal(t, r.URL.String(), "https://api.ivpn.net/v4/servers/stats")
+					return &http.Response{
+						StatusCode: testCase.responseStatus,
+						Status:     http.StatusText(testCase.responseStatus),
+						Body:       testCase.responseBody,
+					}, nil
+				}),
+			}
+
+			data, err := fetchAPI(ctx, client)
+
+			assert.Equal(t, testCase.data, data)
+			if testCase.err != nil {
+				require.Error(t, err)
+				assert.Equal(t, testCase.err.Error(), err.Error())
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}