Adjusted serve-stale test

After the addition of stale-answer-client-timeout a test was broken due
to the following behavior expected by the test.

1. Prime cache data.example txt.
2. Disable authoritative server.
3. Send a query for data.example txt.
4. Recursive server will timeout and answer from cache with stale RRset.
5. Recursive server will activate stale-refresh-time due to the previous
   failure in attempting to refresh the RRset.
6. Send a query for data.example txt.
7. Expect stale answer from cache due to stale-refresh-time
window being active, even if authoritative server is up.

Problem is that in step 4, due to the new option
stale-answer-client-timeout, recursive server will answer with stale
data before the actual fetch completes.

Since the original fetch is still running in background, if we re-enable
the authoritative server during that time, the RRset will actually be
successfully refreshed, and stale-refresh-window will not be activated.

The next queries will fail because they expect the TTL of the RRset to
match the one in the stale cache, not the one just refreshed.

To solve this, we explicitly disable stale-answer-client-timeout for
this test, as it's not the feature we are interested in testing here
anyways.

bin/tests/system/serve-stale/clean.sh

@@ -15,3 +15,4 @@ rm -f */named.run */named.memstats
 rm -f ns*/managed-keys.bind*
 rm -f ns*/named_dump*
 rm -f ns*/named.stats*
+rm -f ns1/named.run.prev

bin/tests/system/serve-stale/ns1/named1.conf.in

@@ -30,6 +30,7 @@ options {
 	max-stale-ttl 3600;
 	stale-answer-ttl 4;
 	stale-answer-enable yes;
+	stale-answer-client-timeout disabled;
 	stale-cache-enable yes;
 	stale-refresh-time 30;
 	servfail-ttl 0;

bin/tests/system/serve-stale/ns1/named2.conf.in

@@ -30,6 +30,7 @@ options {
 	max-stale-ttl 20;
 	stale-answer-ttl 3;
 	stale-answer-enable yes;
+	stale-answer-client-timeout disabled;
 	stale-cache-enable yes;
 	servfail-ttl 0;
 };

bin/tests/system/serve-stale/ns1/named3.conf.in

@@ -30,6 +30,7 @@ options {
 	max-stale-ttl 20;
 	stale-answer-ttl 3;
 	stale-answer-enable yes;
+	stale-answer-client-timeout disabled;
 	stale-cache-enable yes;
 	stale-refresh-time 0;
 	servfail-ttl 0;