Test that transient settings beat persistent ones (elastic#33818)

Transient settings override persistent settings, but in fact all of the tests
that run as part of `:server:test` and `:server:integTest` will pass if the
precedence is changed to be the other way round. This change adds a test that
verifies the precedence is as documented.

server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataTests.java

@@ -29,6 +29,7 @@
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.io.stream.NamedWriteableAwareStreamInput;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
+import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -794,4 +795,12 @@ public static void assertMultiField(Map<String, Object> properties, String field
             "    }\n" +
             "  }\n" +
             "}";
+
+    public void testTransientSettingsOverridePersistentSettings() {
+        final Setting setting = Setting.simpleString("key");
+        final MetaData metaData = MetaData.builder()
+            .persistentSettings(Settings.builder().put(setting.getKey(), "persistent-value").build())
+            .transientSettings(Settings.builder().put(setting.getKey(), "transient-value").build()).build();
+        assertThat(setting.get(metaData.settings()), equalTo("transient-value"));
+    }
 }