Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/tests/subscription_test.go

@@ -0,0 +1,53 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build integration
+// +build integration
+
+package tests
+
+import (
+	"testing"
+
+	miniflux "miniflux.app/v2/client"
+)
+
+func TestDiscoverSubscriptions(t *testing.T) {
+	client := createClient(t)
+	subscriptions, err := client.Discover(testWebsiteURL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(subscriptions) != 1 {
+		t.Fatalf(`Invalid number of subscriptions, got "%v" instead of "%v"`, len(subscriptions), 2)
+	}
+
+	if subscriptions[0].Title != testSubscriptionTitle {
+		t.Fatalf(`Invalid feed title, got "%v" instead of "%v"`, subscriptions[0].Title, testSubscriptionTitle)
+	}
+
+	if subscriptions[0].Type != "atom" {
+		t.Fatalf(`Invalid feed type, got "%v" instead of "%v"`, subscriptions[0].Type, "atom")
+	}
+
+	if subscriptions[0].URL != testFeedURL {
+		t.Fatalf(`Invalid feed URL, got "%v" instead of "%v"`, subscriptions[0].URL, testFeedURL)
+	}
+}
+
+func TestDiscoverSubscriptionsWithInvalidURL(t *testing.T) {
+	client := createClient(t)
+	_, err := client.Discover("invalid")
+	if err == nil {
+		t.Fatal(`Invalid URLs should be rejected`)
+	}
+}
+
+func TestDiscoverSubscriptionsWithNoSubscription(t *testing.T) {
+	client := createClient(t)
+	_, err := client.Discover(testBaseURL)
+	if err != miniflux.ErrNotFound {
+		t.Fatal(`A 404 should be returned when there is no subscription`)
+	}
+}