Pull request: dnsforward: add dnsrewrite tests

Merge in DNS/adguard-home from 2102-dnsrewrite-tests to master

Updates #2102.

Squashed commit of the following:

commit 894ff4baf8378d6e3386e09c416d55241a01f79e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jan 14 14:49:28 2021 +0300

    dnsforward: add dnsrewrite tests

internal/dnsfilter/safebrowsing.go

@@ -46,12 +46,12 @@ func (d *DNSFilter) initSecurityServices() error {
 
 	d.parentalUpstream, err = upstream.AddressToUpstream(d.parentalServer, opts)
 	if err != nil {
-		return err
+		return fmt.Errorf("converting parental server: %w", err)
 	}
 
 	d.safeBrowsingUpstream, err = upstream.AddressToUpstream(d.safeBrowsingServer, opts)
 	if err != nil {
-		return err
+		return fmt.Errorf("converting safe browsing server: %w", err)
 	}
 
 	return nil