unit-test failed on OS X 10.12

Host: macos-1012-80.macstadium.build.10gen.cc
Project: WiredTiger (develop)
Commit: diff: WT-6570 RTS to remove the history store updates with no stop timestamp (#5926)

The updates in the history store with no stop timestamp that are associated with
prepared updates in the data store can lead to ever-increasing history store size
when they are not removed from the history store. Let the RTS to cleanup those
unresolved updates from the history store. | 12 Aug 20 02:07 UTC
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