### Environment:
Elasticsearch version:
```
{
    "number" : "7.15.0",
    "build_flavor" : "default",
    "build_type" : "zip",
    "build_hash" : "79d65f6e357953a5b3cbcc5e2c7c21073d89aa29",
    "build_date" : "2021-09-16T03:05:29.143308416Z",
    "build_snapshot" : false,
    "lucene_version" : "8.9.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
}
```
Plugins installed: []
JVM version :1.8
OS version :Windows 10

### Steps to reproduce:
```
PUT test_plain_highlighter 
{}

POST /test_plain_highlighter/_doc/1
{
  "description": [
    "Lorem Ipsum string Generator that helps to create dummy text for all layout needs.",
    "It has roots in a piece of classical Latin literature from 45 BC, making it search string over 2000 years old."
  ]
}

POST /test_plain_highlighter/_search
{
  "highlight": {
    "order": "score",
    "type": "plain",
    "fields": {
      "*": {}
    },
    "fragment_size": 50
  },
  "query": {
    "match": {
      "description": "search string"
    }
  },
  "_source": ""
}
```

### RECEIVED Highlighted Fragment in response:
```
[
   "Lorem Ipsum <em>string</em> Generator that helps to create",
   " literature from 45 BC, making it <em>search</em> <em>string</em> over 2000 years old."
]
```
### EXPECTED Highlighted Fragment:
```
[
   " literature from 45 BC, making it <em>search</em> <em>string</em> over 2000 years old.",
   "Lorem Ipsum <em>string</em> Generator that helps to create"
]
```
test/framework/src/main/java/org/elasticsearch/search/fetch/HighlighterTestCase.java
import org.elasticsearch.search.fetch.subphase.highlight.HighlightPhase;
import org.elasticsearch.search.fetch.subphase.highlight.Highlighter;
import org.elasticsearch.search.fetch.subphase.highlight.PlainHighlighter;
import org.elasticsearch.search.fetch.subphase.highlight.UnifiedHighlighter;
import java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
public class HighlighterTestCase extends MapperServiceTestCase {
    protected Map<String, Highlighter> getHighlighters() {
        return Map.of(
            "unified",
            new UnifiedHighlighter(),
            "fvh",
            new FastVectorHighlighter(getIndexSettings()),
            "plain",
            new PlainHighlighter()
        );
    }
    /**
     * Runs the highlight phase for a search over a specific document
     * @param mapperService the Mappings to use for highlighting
     * @param doc           a parsed document to highlight
     * @param search        the search to highlight
     */
    protected final Map<String, HighlightField> highlight(MapperService mapperService, ParsedDocument doc, SearchSourceBuilder search)