LUCENE-10054: Make sure to use Lucene90 codec in unit tests (#699)

Before we were using the default Lucene91 codec, so we weren't exercising the
old format.

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene90/Lucene90Codec.java

@@ -164,7 +164,7 @@ public final PointsFormat pointsFormat() {
   }
 
   @Override
-  public final KnnVectorsFormat knnVectorsFormat() {
+  public KnnVectorsFormat knnVectorsFormat() {
     return knnVectorsFormat;
   }
 

lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene90/Lucene90RWCodec.java

@@ -0,0 +1,44 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.backward_codecs.lucene90;
+
+import org.apache.lucene.codecs.KnnVectorsFormat;
+import org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat;
+
+public class Lucene90RWCodec extends Lucene90Codec {
+
+  private final KnnVectorsFormat defaultKnnVectorsFormat;
+  private final KnnVectorsFormat knnVectorsFormat =
+      new PerFieldKnnVectorsFormat() {
+        @Override
+        public KnnVectorsFormat getKnnVectorsFormatForField(String field) {
+          return defaultKnnVectorsFormat;
+        }
+      };
+
+  public Lucene90RWCodec() {
+    this.defaultKnnVectorsFormat =
+        new Lucene90RWHnswVectorsFormat(
+            Lucene90HnswVectorsFormat.DEFAULT_MAX_CONN,
+            Lucene90HnswVectorsFormat.DEFAULT_BEAM_WIDTH);
+  }
+
+  @Override
+  public KnnVectorsFormat knnVectorsFormat() {
+    return knnVectorsFormat;
+  }
+}

lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene90/TestLucene90HnswVectorsFormat.java

@@ -23,12 +23,11 @@
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.tests.index.BaseKnnVectorsFormatTestCase;
-import org.apache.lucene.tests.util.TestUtil;
 
 public class TestLucene90HnswVectorsFormat extends BaseKnnVectorsFormatTestCase {
   @Override
   protected Codec getCodec() {
-    return TestUtil.getDefaultCodec();
+    return new Lucene90RWCodec();
   }
 
   public void testToString() {