Improve watcher rolling upgrade tests

Relates to elastic#33185

Author: martijnvg

x-pack/qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/UpgradeClusterClientYamlTestSuiteIT.java

@@ -46,6 +46,7 @@ public void waitForWatcher() throws Exception {
             Response response = client().performRequest(new Request("GET", "_watcher/stats"));
             Map<String, Object> responseBody = entityAsMap(response);
             List<?> stats = (List<?>) responseBody.get("stats");
+            assertThat(stats.size(), equalTo(3));
             for (Object stat : stats) {
                 Map<?, ?> statAsMap = (Map<?, ?>) stat;
                 assertThat(statAsMap.get("watcher_state"), equalTo("started"));

x-pack/qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/60_watcher.yml

@@ -27,6 +27,14 @@
   - match: { found : true}
   - match: { _id: "my_watch" }
 
+  - do:
+      cluster.health:
+        index: .watches
+        wait_for_status: green
+        wait_for_no_relocating_shards: true
+        wait_for_events: languid
+        timeout: 1m
+
   # execute watch
   - do:
       watcher.execute_watch: