Follow on work from WT-5916, after the call to __wt_txn_begin on line 555, of txn_ckpt.c we should add a new timing stress configuration and additionally configure it for a few of our
tests, e.g. test_timestamp_abort and test_format.

The timing stress could delay the checkpoint by say 1000 microseconds in that location.

I did a similar change in WT-6371.