Fix timeout in test_checkpoint_snapshot04

unit-test failed on ~ Ubuntu 18.04 zSeries

Host: ubuntu1804-z-11.dallasisv.build.10gen.cc
Project: WiredTiger (develop)
Commit: diff: WT-7667 Fix workgen json output

The following changes have been done to the output JSON format:

• "workgen" entries are now packed into array
• "localTime" element belongs to entry and should be placed inside
• "version" does not belong to any entry and should be placed into the root element

Also, in order to reduce complexity and improve readability of Monitor::run, several methods were extracted from the function. | 17 Jun 21 07:44 UTC
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