Pull request: all: rm var shadowing vol. 3

Updates #2803.

Squashed commit of the following:

commit ce711ae2e08c3714a4e0c6d177c02f7801c05fce
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:58:40 2021 +0300

    all: imp code, docs

commit e91094e461893c27eda879b33b5ed2c7085510df
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:42:14 2021 +0300

    all: rm var shadowing vol. 3

internal/dnsforward/dnsforward_test.go

@@ -46,8 +46,8 @@ func startDeferStop(t *testing.T, s *Server) {
 	require.Nilf(t, err, "failed to start server: %s", err)
 
 	t.Cleanup(func() {
-		err := s.Stop()
-		require.Nilf(t, err, "dns server failed to stop: %s", err)
+		serr := s.Stop()
+		require.Nilf(t, serr, "dns server failed to stop: %s", serr)
 	})
 }