Pull request 2015: AG-23168-imp-updater-tests

Squashed commit of the following:

commit 402ac873839fcf4dfb762872497b740bd55b2dcc
Merge: 209e1b171 af38476ef
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Thu Sep 21 16:46:47 2023 +0300

    Merge branch 'master' into AG-23168-imp-updater-tests

commit 209e1b171cb4c55d177ef5a4198cf24c2bc41195
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Thu Sep 21 14:09:43 2023 +0300

    updater: fix windows build

commit d112ca53540e0c999fe126442e8c526ff7e33bf5
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Sep 20 18:57:54 2023 +0300

    all: imp tests, docs

commit f940724f81a90af6c07de523c8fcce66240dbec9
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Sep 20 16:07:48 2023 +0300

    all: imp tests

commit 8915818e81360a7f6ff5b652a983ca034ad6d0c4
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Tue Sep 19 19:33:20 2023 +0300

    all: add test

commit c1cb8df2b056cefc5d7f1b32c089cd0ede0af9d2
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Thu Sep 14 19:12:47 2023 +0300

    updater: imp tests

internal/aghtest/aghtest.go

@@ -4,10 +4,14 @@ package aghtest
 import (
 	"crypto/sha256"
 	"io"
+	"net/http"
+	"net/http/httptest"
 	"net/netip"
+	"net/url"
 	"testing"
 
 	"github.com/AdguardTeam/golibs/log"
+	"github.com/stretchr/testify/require"
 )
 
 const (
@@ -51,3 +55,19 @@ func HostToIPs(host string) (ipv4, ipv6 netip.Addr) {
 
 	return netip.AddrFrom4([4]byte(hash[:4])), netip.AddrFrom16([16]byte(hash[4:20]))
 }
+
+// StartHTTPServer is a helper that starts the HTTP server, which is configured
+// to return data on every request, and returns the client and server URL.
+func StartHTTPServer(t testing.TB, data []byte) (c *http.Client, u *url.URL) {
+	t.Helper()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write(data)
+	}))
+	t.Cleanup(srv.Close)
+
+	u, err := url.Parse(srv.URL)
+	require.NoError(t, err)
+
+	return srv.Client(), u
+}