In WT-16037 a new Python runner flag "--skip-tests-in-file" was introduced to facilitate skipping tests listed in "test/suite/hook_disagg.fail" for code coverage tests. Prior to that, there's another equivalent implemention using Evergreen variable "unit_test_ignore".

In order to avoid diversion and reduce maintenance cost, we should consider merge these 2 implementations, presumably to the "--skip-tests-in-file" Python runner flag since it's more portable than the Evergreen variable (only usable for Evergreen tasks).

Outcomes:

• All places currently use Evergreen variable "unit_test_ignore" in test/evergreen.yml and test/evergreen_develop.yml are replaced by the "--skip-tests-in-file" Python runner flag
• The use of "unit_test_ignore" variable is removed from the "unit test" Evergreen function.