Prevent tiered dhandles from being collected prematurely (entry->tiered->bstorage != NULL assertion in __wt_tiered_pop_work)

Test failure

ERROR: test_sweep01.test_sweep01.test_ops(row) (subunit.RemotedTestCase)

test_sweep01.test_sweep01.test_ops(row)

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testtools.testresult.real._StringException: lost connection during test 'test_sweep01.test_sweep01.test_ops(row)'

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Ran 9334 tests in 1490.701s

Link: https://evergreen.mongodb.com/task_log_raw/wiredtiger_ubuntu2004_unit_test_hook_tiered_6fddc2ea2b9c07b3a0e7dbdb51f46d601ee32ca0_22_11_28_20_01_46/0?type=T#L24

Verbose Error message

[1669667349:731366][35420:0x7fe10d3be700], test_sweep01.test_sweep01.test_ops(row), tiered-server: [WT_VERB_DEFAULT][ERROR]: __wt_tiered_pop_work, 118: entry->tiered->bstorage != NULL

[1669667349:731516][35420:0x7fe10d3be700], test_sweep01.test_sweep01.test_ops(row), tiered-server: [WT_VERB_DEFAULT][ERROR]: __wt_abort, 28: aborting WiredTiger library

Link: https://evergreen.mongodb.com/task_log_raw/wiredtiger_ubuntu2004_unit_test_hook_tiered_6fddc2ea2b9c07b3a0e7dbdb51f46d601ee32ca0_22_11_28_20_01_46/0?type=T#L80