LUCENE-9935: Clone term vectors reader for merges (#182)

The newly added assertion in the bulk-merge logic doesn't always hold 
because we do not create a new instance of
Lucene90CompressingTermVectorsReader for merges and that reader can be
accessed in tests (as long as it happens on the same thread).

This change clones a new term vectors reader for merges.

lucene/core/src/java/org/apache/lucene/codecs/lucene90/compressing/Lucene90CompressingTermVectorsReader.java

@@ -304,6 +304,11 @@ public final class Lucene90CompressingTermVectorsReader extends TermVectorsReade
     return new Lucene90CompressingTermVectorsReader(this);
   }
 
+  @Override
+  public TermVectorsReader getMergeInstance() {
+    return new Lucene90CompressingTermVectorsReader(this);
+  }
+
   private static RandomAccessInput slice(IndexInput in) throws IOException {
     final int length = in.readVInt();
     final byte[] bytes = new byte[length];