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/MultiFeatureMigrationIT.java
}
    @Override
    protected Collection<Class<? extends Plugin>> nodePlugins() {
        List<Class<? extends Plugin>> plugins = new ArrayList<>(super.nodePlugins());
        plugins.add(TestPlugin.class);
        plugins.add(SecondPlugin.class);
        plugins.add(ReindexPlugin.class);
        return plugins;
    }
    
    private static final String SECOND_FEATURE_NAME = "B-test-feature";
    private static final String ORIGIN = MultiFeatureMigrationIT.class.getSimpleName();
    private static final String VERSION_META_KEY = "version";
    static final int SECOND_FEATURE_IDX_FLAG_VALUE = 0;
    public void testMultipleFeatureMigration() throws Exception {
        
        createSystemIndexForDescriptor(INTERNAL_MANAGED);
        createSystemIndexForDescriptor(INTERNAL_UNMANAGED);
        createSystemIndexForDescriptor(EXTERNAL_MANAGED);
        createSystemIndexForDescriptor(EXTERNAL_UNMANAGED);
        
        createSystemIndexForDescriptor(SECOND_FEATURE_IDX_DESCIPTOR);
        ensureGreen();
        CountDownLatch hooksCalled = new CountDownLatch(4);
        SetOnce<Boolean> preMigrationHookCalled = new SetOnce<>();
        SetOnce<Boolean> postMigrationHookCalled = new SetOnce<>();
        SetOnce<Boolean> secondPluginPreMigrationHookCalled = new SetOnce<>();
        SetOnce<Boolean> secondPluginPostMigrationHookCalled = new SetOnce<>();
        TestPlugin.preMigrationHook.set(clusterState -> {