unit-test failed on OS X 10.12

Host: macos-1012-100.macstadium.build.10gen.cc
Project: WiredTiger (develop)
Commit: diff: WT-6710 Change default transaction isolation to snapshot (#6192)

The default session/transaction isolation is changed from read-committed to snapshot
for all the user and internal sessions. The meta checkpoint internal session still operates
under the read-committed isolation mode. Metadata updates are not checked against
concurrent access. | 22 Dec 20 05:04 UTC
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