Sentinel test: set less time sensitive defaults.

This commit sets the failover timeout to 30 seconds instead of the 180
seconds default, and allows to reconfigure multiple slaves at the same
time.

This makes tests less sensible to timing, with the result that there are
less false positives due to normal behaviors that require time to
succeed or to be retried.

However the long term solution is probably some way in order to detect
when a test failed because of timing issues (for example split brain
during leader election) and retry it.

tests/sentinel-tests/includes/init-tests.tcl

@@ -30,6 +30,8 @@ test "(init) Sentinels can start monitoring a master" {
     foreach_sentinel_id id {
         assert {[S $id sentinel master mymaster] ne {}}
         S $id SENTINEL SET mymaster down-after-milliseconds 2000
+        S $id SENTINEL SET mymaster failover-timeout 20000
+        S $id SENTINEL SET mymaster parallel-syncs 10
     }
 }