Fail rest tests on yaml files (#24740)

We've switched to supporting only `yml` files but anyone who didn't
notice will commit a `yaml` file which won't be executed
which is bad because it is easy not to notice. The test to catch this is
simple enough that I think it is worth adding just to warn folks about
their mistake.

test/framework/src/main/java/org/elasticsearch/test/rest/yaml/ESClientYamlSuiteTestCase.java

@@ -200,6 +200,8 @@ public abstract class ESClientYamlSuiteTestCase extends ESRestTestCase {
                 Files.walk(path).forEach(file -> {
                     if (file.toString().endsWith(".yml")) {
                         addSuite(root, file, files);
+                    } else if (file.toString().endsWith(".yaml")) {
+                        throw new IllegalArgumentException("yaml files are no longer supported: " + file);
                     }
                 });
             } else {