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
);
        } else {
            
            createRequest.setSettings(
                Settings.builder()
                    .put("index.version.created", Version.CURRENT)
                    .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 0)
                    .build()
            );
        }
        if (descriptor.getMappings() == null) {
            createRequest.setMapping(createSimpleMapping(false, descriptor.isInternal()));
        }
        CreateIndexResponse response = createRequest.get();
        Assert.assertTrue(response.isShardsAcknowledged());
        List<IndexRequestBuilder> docs = new ArrayList<>(INDEX_DOC_COUNT);
        for (int i = 0; i < INDEX_DOC_COUNT; i++) {
            docs.add(ESIntegTestCase.client().prepareIndex(indexName).setId(Integer.toString(i)).setSource("some_field", "words words"));
        }
        indexRandom(true, docs);
        IndicesStatsResponse indexStats = ESIntegTestCase.client().admin().indices().prepareStats(indexName).setDocs(true).get();
        Assert.assertThat(indexStats.getIndex(indexName).getTotal().getDocs().getCount(), is((long) INDEX_DOC_COUNT));
    }
    static Settings createSimpleSettings(Version creationVersion, int flagSettingValue) {
        return Settings.builder()
            .put(IndexMetadata.INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 1)
            .put(IndexMetadata.INDEX_NUMBER_OF_REPLICAS_SETTING.getKey(), 0)
            .put(FlAG_SETTING_KEY, flagSettingValue)
            .put("index.version.created", creationVersion)
            .build();