MB-61702: Add failover start test condition

To arbitrarily delay failovers from starting

Change-Id: Ia4f06cc4e79338d3aadca7fe2b613219468a2a32
Reviewed-on: https://review.couchbase.org/c/ns_server/+/209283
Tested-by: Ben Huddleston <ben.huddleston@couchbase.com>
Reviewed-by: Peter Searby <peter.searby@couchbase.com>
Well-Formed: Restriction Checker
Well-Formed: Build Bot <build@couchbase.com>

src/failover.erl

@@ -172,6 +172,8 @@ orchestrate(Nodes, Options) when Nodes =/= [] ->
     ale:info(?USER_LOGGER, "Starting failing over ~p", [Nodes]),
     master_activity_events:note_failover(Nodes),
 
+    ok = testconditions:check_test_condition(failover_start),
+
     Res =
         case config_sync_and_orchestrate(Nodes, Options) of
             {done, ErrorNodes, UnsafeNodes} ->