`concurrentMix` test seems to fail sporadically

[hatyo]

One of the PRB jobs failed because of a failure in test LuceneIndexMaintenanceTest#concurrentMix.

LuceneIndexMaintenanceTest > concurrentMix(boolean, PartitionCount) > [3] !isSynthetic, MULTIPLE FAILED
    org.opentest4j.AssertionFailedError: [Field { 'group' None}, lucene_stored(Field { 'int_value' None}), lucene_text(Field { 'text_value' None}), lucene_sorted(Field { 'timestamp' None})] group 3 (1):3 ==> expected: <[(1, 1200), (1, 1201), (1, 1203), (1, 1206), (1, 1208), (1, 1209), (1, 1210), (1, 1211), (1, 1214), (1, 1216), (1, 1217), (1, 1218), (1, 1219), (1, 1220), (1, 1221), (1, 1223), (1, 1226), (1, 1227), (1, 1228), (1, 1229), (1, 1231), (1, 1234), (1, 1236), (1, 1237), (1, 1238), (1, 1239), (1, 1240), (1, 1241), (1, 1243), (1, 1244), (1, 1245), (1, 1247), (1, 1248), (1, 1250), (1, 1251), (1, 1254), (1, 1256), (1, 1257), (1, 1258), (1, 1259), (1, 1260), (1, 1261), (1, 1263), (1, 1266), (1, 1268), (1, 1269), (1, 1270), (1, 1271), (1, 1272), (1, 1273), (1, 1275), (1, 1276), (1, 1277), (1, 1279), (1, 1281), (1, 1282), (1, 1283), (1, 1284), (1, 1285), (1, 1286), (1, 1288), (1, 1291), (1, 1293), (1, 1294), (1, 1295), (1, 1296), (1, 1297), (1, 1298), (1, 1300), (1, 1303), (1, 1305), (1, 1306), (1, 1307), (1, 1308), (1, 1311), (1, 1313), (1, 1314), (1, 1315), (1, 1316), (1, 1317), (1, 1318), (1, 1320), (1, 1323), (1, 1325), (1, 1326), (1, 1327), (1, 1328), (1, 1329), (1, 1330), (1, 1332), (1, 1335), (1, 1337), (1, 1338), (1, 1339), (1, 1340), (1, 1343), (1, 1345), (1, 1346), (1, 1347), (1, 1348)]> but was: <[(1, 1200), (1, 1201), (1, 1203), (1, 1206), (1, 1208), (1, 1209), (1, 1210), (1, 1211), (1, 1214), (1, 1216), (1, 1217), (1, 1218), (1, 1219), (1, 1220), (1, 1221), (1, 1223), (1, 1226), (1, 1227), (1, 1228), (1, 1229), (1, 1231), (1, 1234), (1, 1236), (1, 1237), (1, 1238), (1, 1239), (1, 1240), (1, 1241), (1, 1243), (1, 1244), (1, 1245), (1, 1247), (1, 1248), (1, 1250), (1, 1251), (1, 1254), (1, 1256), (1, 1257), (1, 1258), (1, 1259), (1, 1260), (1, 1261), (1, 1263), (1, 1266), (1, 1268), (1, 1269), (1, 1270), (1, 1271), (1, 1272), (1, 1273), (1, 1275), (1, 1276), (1, 1277), (1, 1279), (1, 1281), (1, 1282), (1, 1283), (1, 1284), (1, 1285), (1, 1286), (1, 1288), (1, 1291), (1, 1293), (1, 1294), (1, 1295), (1, 1296), (1, 1297), (1, 1298), (1, 1300), (1, 1303), (1, 1305), (1, 1306), (1, 1307), (1, 1308), (1, 1308), (1, 1311), (1, 1313), (1, 1314), (1, 1315), (1, 1315), (1, 1316), (1, 1317), (1, 1318), (1, 1320), (1, 1323), (1, 1325), (1, 1326), (1, 1327), (1, 1328), (1, 1329), (1, 1330), (1, 1332), (1, 1332), (1, 1335), (1, 1337), (1, 1338), (1, 1339), (1, 1340), (1, 1346), (1, 1347)]>
        at app//org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
        at app//org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
        at app//org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
        at app//org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:188)
        at app//org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:1167)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexTestValidator.validateDocsInPartition(LuceneIndexTestValidator.java:342)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexTestValidator.validatePartitionedGroup(LuceneIndexTestValidator.java:196)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexTestValidator.validate(LuceneIndexTestValidator.java:139)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexTestValidator.validate(LuceneIndexTestValidator.java:102)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexTestDataModel.validate(LuceneIndexTestDataModel.java:301)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexMaintenanceTest.concurrentTestWithinTransaction(LuceneIndexMaintenanceTest.java:1356)
        at app//com.apple.foundationdb.record.lucene.LuceneIndexMaintenanceTest.concurrentMix(LuceneIndexMaintenanceTest.java:1245)