[TEST] Correctly verify that aliases were deleted in MlJobIT

This fixes `testDeleteJobAfterMissingAliases` to not fail randomly.
The reason the test was failing is that at some point some aliases
are deleted and the cat-aliases API is called to verify they were
indeed deleted. This was checked by asserting an
index_not_found_exception was thrown by the cat-aliases request.
This was some times working as there were no other aliases. However,
that depends on whether other x-pack features had time to create their
infrastructure. For example, security creates an alias. When other
aliases had the time to be created, the cat-aliases request does not
fail and the test fails.

This commit simply changes the verification that the read/write
aliases were deleted by replacing the cat-aliases request with
two single get-alias requests.

Original commit: elastic/x-pack-elasticsearch@fe2c7b0cb4

plugin/src/test/java/org/elasticsearch/xpack/ml/integration/MlJobIT.java

@@ -444,8 +444,9 @@ public class MlJobIT extends ESRestTestCase {
         response = client().performRequest("delete", indexName + "/_alias/" + writeAliasName);
         assertEquals(200, response.getStatusLine().getStatusCode());
 
-        // check alias was deleted
-        expectThrows(ResponseException.class, () -> client().performRequest("get", "_cat/aliases"));
+        // check aliases were deleted
+        expectThrows(ResponseException.class, () -> client().performRequest("get", indexName + "/_alias/" + readAliasName));
+        expectThrows(ResponseException.class, () -> client().performRequest("get", indexName + "/_alias/" + writeAliasName));
 
         response = client().performRequest("delete", MachineLearning.BASE_PATH + "anomaly_detectors/" + jobId);
         assertThat(response.getStatusLine().getStatusCode(), equalTo(200));