Pull request 1992: 3701-fallback-dns-check

Squashed commit of the following:

commit a5191def0b950cb7ab1ea737e9c8c6e8b65ec720
Merge: 7d127bfca 5bd452516
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Sep 6 12:21:08 2023 +0300

    Merge branch 'master' into 3701-fallback-dns-check

commit 7d127bfca54ac940c611681e6f48b89881491602
Author: Ildar Kamalov <ik@adguard.com>
Date:   Tue Sep 5 13:21:35 2023 +0300

    client: add fallback_dns to the upstream test

commit 970da7215503b5f05c53fcad47c715a96d9fc3c4
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Mon Sep 4 19:23:35 2023 +0300

    all: add fallback dns check

internal/dnsforward/http_test.go

@@ -560,6 +560,23 @@ func TestServer_HandleTestUpstreamDNS(t *testing.T) {
 			hostsUps: "OK",
 		},
 		name: "etc_hosts",
+	}, {
+		body: map[string]any{
+			"fallback_dns": []string{goodUps},
+		},
+		wantResp: map[string]any{
+			goodUps: "OK",
+		},
+		name: "fallback_success",
+	}, {
+		body: map[string]any{
+			"fallback_dns": []string{badUps},
+		},
+		wantResp: map[string]any{
+			badUps: `couldn't communicate with upstream: exchanging with ` +
+				badUps + ` over tcp: dns: id mismatch`,
+		},
+		name: "fallback_broken",
 	}}
 
 	for _, tc := range testCases {