Fix BWC test generation after mondernizing LineDocFile (apache#13021)

The changes on apache#12929 broke the generation code for BWC indices
since they are expecting vertain fields created by LineDocFile.
Yet, this change adds some sanity checks that run with unittest to ensure
the BWC generation is at least readable with the current version.

Relates to apache#12929

lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestBackwardsCompatibility.java

@@ -205,11 +205,20 @@ private Path getIndexDir() {
   }
 
   public void testCreateMoreTermsIndex() throws Exception {
-
     Path indexDir = getIndexDir().resolve("moreterms");
     Files.deleteIfExists(indexDir);
-    Directory dir = newFSDirectory(indexDir);
+    try (Directory dir = newFSDirectory(indexDir)) {
+      createMoreTermsIndex(dir);
+    }
+  }
+
+  public void testCreateMoreTermsIndexInternal() throws Exception {
+    try (Directory dir = newDirectory()) {
+      createMoreTermsIndex(dir);
+    }
+  }
 
+  private void createMoreTermsIndex(Directory dir) throws Exception {
     LogByteSizeMergePolicy mp = new LogByteSizeMergePolicy();
     mp.setNoCFSRatio(1.0);
     mp.setMaxCFSSegmentSizeMB(Double.POSITIVE_INFINITY);
@@ -220,22 +229,48 @@ public void testCreateMoreTermsIndex() throws Exception {
         new IndexWriterConfig(analyzer).setMergePolicy(mp).setUseCompoundFile(false);
     IndexWriter writer = new IndexWriter(dir, conf);
     LineFileDocs docs = new LineFileDocs(new Random(0));
+    Field docIdDV = null;
+    Field titleDVField = null;
     for (int i = 0; i < 50; i++) {
-      writer.addDocument(docs.nextDoc());
+      Document doc = docs.nextDoc();
+      if (docIdDV == null) {
+        docIdDV = new NumericDocValuesField("docid_intDV", 0);
+        doc.add(docIdDV);
+      }
+      docIdDV.setLongValue(doc.getField("docid_int").numericValue().longValue());
+      if (titleDVField == null) {
+        titleDVField = new SortedDocValuesField("titleDV", new BytesRef());
+        doc.add(titleDVField);
+      }
+      titleDVField.setBytesValue(new BytesRef(doc.getField("title").stringValue()));
+      writer.addDocument(doc);
     }
     docs.close();
     writer.close();
-    dir.close();
+    try (DirectoryReader reader = DirectoryReader.open(dir)) {
+      searchExampleIndex(reader); // make sure we can search it
+    }
   }
 
   // gradlew test -Ptestmethod=testCreateSortedIndex -Ptests.codec=default
   // -Ptests.useSecurityManager=false -Ptests.bwcdir=/tmp/sorted --tests TestBackwardsCompatibility
   public void testCreateSortedIndex() throws Exception {
-
     Path indexDir = getIndexDir().resolve("sorted");
     Files.deleteIfExists(indexDir);
-    Directory dir = newFSDirectory(indexDir);
+    try (Directory dir = newFSDirectory(indexDir)) {
+      createSortedIndex(dir);
+    }
+  }
 
+  public void testCreateSortedIndexInternal() throws Exception {
+    // this runs without the -Ptests.bwcdir=/tmp/sorted to make sure we can actually index and
+    // search the created index
+    try (Directory dir = newDirectory()) {
+      createSortedIndex(dir);
+    }
+  }
+
+  public void createSortedIndex(Directory dir) throws Exception {
     LogByteSizeMergePolicy mp = new LogByteSizeMergePolicy();
     mp.setNoCFSRatio(1.0);
     mp.setMaxCFSSegmentSizeMB(Double.POSITIVE_INFINITY);
@@ -248,11 +283,14 @@ public void testCreateSortedIndex() throws Exception {
     conf.setUseCompoundFile(false);
     conf.setIndexSort(new Sort(new SortField("dateDV", SortField.Type.LONG, true)));
     IndexWriter writer = new IndexWriter(dir, conf);
-    LineFileDocs docs = new LineFileDocs(random());
+    LineFileDocs docs = new LineFileDocs(new Random(0));
     SimpleDateFormat parser = new SimpleDateFormat("yyyy-MM-dd", Locale.ROOT);
     parser.setTimeZone(TimeZone.getTimeZone("UTC"));
     ParsePosition position = new ParsePosition(0);
+    Field docIdDV = null;
     Field dateDVField = null;
+    Field titleDVField = null;
+
     for (int i = 0; i < 50; i++) {
       Document doc = docs.nextDoc();
       String dateString = doc.get("date");
@@ -270,14 +308,28 @@ public void testCreateSortedIndex() throws Exception {
         doc.add(dateDVField);
       }
       dateDVField.setLongValue(date.getTime());
+      if (docIdDV == null) {
+        docIdDV = new NumericDocValuesField("docid_intDV", 0);
+        doc.add(docIdDV);
+      }
+
+      docIdDV.setLongValue(doc.getField("docid_int").numericValue().longValue());
+      if (titleDVField == null) {
+        titleDVField = new SortedDocValuesField("titleDV", new BytesRef());
+        doc.add(titleDVField);
+      }
+      titleDVField.setBytesValue(new BytesRef(doc.getField("title").stringValue()));
       if (i == 250) {
         writer.commit();
       }
       writer.addDocument(doc);
     }
     writer.forceMerge(1);
     writer.close();
-    dir.close();
+
+    try (DirectoryReader reader = DirectoryReader.open(dir)) {
+      searchExampleIndex(reader); // make sure we can search it
+    }
   }
 
   private void updateNumeric(IndexWriter writer, String id, String f, String cf, long value)