Fix global checkpoints test bug

This commit fixes a test bug in a global checkpoints integration
test. Namely, if the replica shard is slow to start and is peer
recovered from the primary, it will not have the expected global
checkpoint due to these not being persisted and transferred on recovery.

Relates #18755

Author: jasontedor

core/src/test/java/org/elasticsearch/index/seqno/CheckpointsIT.java

@@ -61,16 +61,19 @@ public void testCheckpointsAdvance() throws Exception {
                     XContentHelper.toString(shardStats.getSeqNoStats(),
                         new ToXContent.MapParams(Collections.singletonMap("pretty", "false"))));
                 final Matcher<Long> localCheckpointRule;
+                final Matcher<Long> globalCheckpointRule;
                 if (shardStats.getShardRouting().primary()) {
                     localCheckpointRule = equalTo(numDocs - 1L);
+                    globalCheckpointRule = equalTo(numDocs - 1L);
                 } else {
-                    // nocommit:  recovery doesn't transfer local checkpoints yet (we don't persist them in lucene).
+                    // nocommit: recovery doesn't transfer checkpoints yet (we don't persist them in lucene).
                     localCheckpointRule = anyOf(equalTo(numDocs - 1L), equalTo(SequenceNumbersService.NO_OPS_PERFORMED));
+                    globalCheckpointRule = anyOf(equalTo(numDocs - 1L), equalTo(SequenceNumbersService.UNASSIGNED_SEQ_NO));
                 }
                 assertThat(shardStats.getShardRouting() + " local checkpoint mismatch",
                     shardStats.getSeqNoStats().getLocalCheckpoint(), localCheckpointRule);
                 assertThat(shardStats.getShardRouting() + " global checkpoint mismatch",
-                    shardStats.getSeqNoStats().getGlobalCheckpoint(), equalTo(numDocs - 1L));
+                    shardStats.getSeqNoStats().getGlobalCheckpoint(), globalCheckpointRule);
                 assertThat(shardStats.getShardRouting() + " max seq no mismatch",
                     shardStats.getSeqNoStats().getMaxSeqNo(), equalTo(numDocs - 1L));
             }