There's some cleanup to be done in `FeatureMigrationIT` and related classes.

- [x] Convert `static` `preMigrationHook` and `postMigrationHook` to use instance members, retrieved from the `internalCluster()`
- [x] Use Hamcrest matchers as described [in this comment](https://github.com/elastic/elasticsearch/pull/84192/files#r812224067) for better failure output
- [x] Rename `createSimpleSettings` to `createSettings`
- [x] Rename `createSimpleMapping` to `createMapping`
- [x] add support for unknown fields https://github.com/elastic/elasticsearch/pull/84258#pullrequestreview-890890422
modules/reindex/src/internalClusterTest/java/org/elasticsearch/migration/AbstractFeatureMigrationIntegTest.java
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.indices.AssociatedIndexDescriptor;
import org.elasticsearch.indices.SystemIndexDescriptor;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.SystemIndexPlugin;
import org.elasticsearch.test.ESIntegTestCase;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xcontent.json.JsonXContent;
import org.junit.Assert;
import org.junit.Before;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.BiConsumer;
import java.util.function.Function;
import static org.hamcrest.Matchers.containsInAnyOrder;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.is;
public abstract class AbstractFeatureMigrationIntegTest extends ESIntegTestCase {