linked_list-test: increase write timeouts to 60s
linked_list-test: increase write timeouts to 60s

This makes them consistent with the test's read timeouts, and will hopefully
help deflake the test (the flaky test dashboard shows that all recent
failures were due to writes timing out).

I looped a RELEASE build of the test in slow mode 1000 times. Without the
patch there were 104 failures, all of them write timeouts. With the patch there
was only one failure (a write timeout).

Change-Id: I239de15a59f7e9911e4a3fd4d809ec89d644f66e
Reviewed-on: http://gerrit.cloudera.org:8080/11186
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>
Tested-by: Kudu Jenkins

helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu