Add check for knn availability

Author: carlosdelest

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/plugin/KnnFunctionIT.java

@@ -14,6 +14,7 @@
 import org.elasticsearch.xcontent.XContentFactory;
 import org.elasticsearch.xpack.esql.EsqlTestUtils;
 import org.elasticsearch.xpack.esql.action.AbstractEsqlIntegTestCase;
+import org.elasticsearch.xpack.esql.action.EsqlCapabilities;
 import org.junit.Before;
 
 import java.io.IOException;
@@ -110,6 +111,8 @@ public void testKnnNonPushedDown() {
 
     @Before
     public void setup() throws IOException {
+        assumeTrue("Needs KNN support", EsqlCapabilities.Cap.KNN_FUNCTION.isEnabled());
+
         var indexName = "test";
         var client = client().admin().indices();
         XContentBuilder mapping = XContentFactory.jsonBuilder()