[TEST] Use better REST endpoints for testing watch actions

This is the x-pack side of https://github.com/elastic/elasticsearch/pull/24437

It changes two things, for the disable tests, it uses a valid endpoint instead
of a previously invalid endpoint that happened to return a 400 because the
endpoint was bad, regardless of if watcher was disabled.

The other change is to create the watches index by putting a watch using the
correct API, rather than manually creating the index. This is because
`RestHijackOperationAction` hijacks operations like this and stops accessing the
endpoint in a regular manner.

Original commit: elastic/x-pack-elasticsearch@3be78d9aea

plugin/src/test/java/org/elasticsearch/xpack/watcher/WatcherPluginDisableTests.java

@@ -70,7 +70,7 @@ public class WatcherPluginDisableTests extends ESIntegTestCase {
 
     public void testRestEndpoints() throws Exception {
         try {
-            getRestClient().performRequest("GET", "/_xpack/watcher");
+            getRestClient().performRequest("GET", "/_xpack/watcher/watch/my-watch");
             fail("request should have failed");
         } catch(ResponseException e) {
             assertThat(e.getResponse().getStatusLine().getStatusCode(), is(HttpStatus.SC_BAD_REQUEST));

plugin/src/test/resources/rest-api-spec/test/watcher/get_watch/20_missing.yml

@@ -4,12 +4,38 @@
       cluster.health:
           wait_for_status: yellow
 
-  # ensure index exists, but ignore if it does already
-  # this test should ensure the watch is missing, while the index is there
+  # ensure index exists by creating a different watch
   - do:
-      indices.create:
-        index: .watches
-        ignore: 400
+      xpack.watcher.put_watch:
+        id: "other"
+        body:  >
+          {
+            "trigger": {
+              "schedule": {
+                "hourly": {
+                  "minute": [ 0, 5 ]
+                  }
+                }
+            },
+            "input": {
+              "simple": {
+                "payload": {
+                  "send": "yes"
+                }
+              }
+            },
+            "condition": {
+              "always": {}
+            },
+            "actions": {
+                "test_index": {
+                  "index": {
+                    "index": "test",
+                    "doc_type": "test2"
+                  }
+                }
+              }
+            }
 
   - do:
       catch: missing