make-check-test failed on * OS X 10.14 CMake

Host: macos-1014-131.macstadium.build.10gen.cc
Project: WiredTiger (mongodb-4.4)
Commit: diff: WT-8708 Fix timestamp usage error in test/checkpoint (#7582) (#7691)

Don't read g.ts_stable until it's actually been set by set_stable(), otherwise we can try and set the oldest timestamp after the stable timestamp.

(cherry picked from commit d4b0ad6cacb874fdc20bcc76311d789dd5a01441)

Co-authored-by: Keith Bostic <keith.bostic@mongodb.com> | 22 Mar 22 02:18 UTC
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Task Logs (make-check-test)