Add bw tests for block-tree with inlined metadata. (apache#13527)

The backport of apache#13524 found a hole in the testing of `Lucene40BlockTreeTerms`
for versions before we moved metadata to its own file. This PR adds explicit bw
testing for this version. Adding the correct if/else statements made the code
extremely complicated so I opted for restoring the file as it was at the time
when we bumped the version.

This also fixes the bug that we introduced in apache#13524.

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene40/blocktree/FieldReader.java

@@ -93,6 +93,8 @@ public final class FieldReader extends Terms {
       final IndexInput clone = indexIn.clone();
       clone.seek(indexStartFP);
       fstMetadata = readMetadata(clone, ByteSequenceOutputs.getSingleton());
+      // FST bytes actually only start after the metadata.
+      indexStartFP = clone.getFilePointer();
     } else {
       fstMetadata = readMetadata(metaIn, ByteSequenceOutputs.getSingleton());
     }