unit-test failed on OS X 10.14

Host: macos-1014-68.macstadium.build.10gen.cc
Project: WiredTiger (develop)
Commit: diff: WT-8836 Flush objects after checkpoint gen advances. (#7635)

• WT-8836 Flush objects after checkpoint gen advances.

• Drain work on shutdown and signal after checkpoint. Update a unit test.

• Typo and turn off verbose in test.

• First pass at python tests and 'sync=off' for flush tier.

• Make sure config is set in tiered13

• Move tiered_storage_destroy after final checkpoint.

• Fix test_tiered10.py

• Fix checkpoint to allow flushing to proceed without dirty data.

• Fix python tiered tests for new flush_tier usage.

• Add debug_mode for flush tier calling checkpoint to avoid testing
  gymnastics.

• Remove some verbose lines.

• Simplify flush generation to boolean indicator. | 11 Mar 22 21:38 UTC
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