QueryStageCollectionScan test failing

[IanWhalen]

https://jira.mongodb.org/browse/SERVER-33409

[db_test:QueryStageCollectionScan] 2018-02-14T17:51:37.268+0000 2018-02-14T17:51:37.267+0000 I -        [testsuite] QueryStageCollectionScan       | tests:    8 | fails:    2 | assert calls:          0 | time secs:  0.096
[db_test:QueryStageCollectionScan] 2018-02-14T17:51:37.268+0000 	QueryStageCollectionScan::QueryStageCollscanBasicBackward	Expected numObj() == countResults(CollectionScanParams::BACKWARD, BSONObj()) (50 == 0) @src/mongo/dbtests/query_stage_collscan.cpp:175
[db_test:QueryStageCollectionScan] 2018-02-14T17:51:37.268+0000 	QueryStageCollectionScan::QueryStageCollscanInvalidateUpcomingObjectBackward	Expected numObj() == count (50 == 11) @src/mongo/dbtests/query_stage_collscan.cpp:399