Currently, the selected_tests_gen task only runs tasks on the enterprise-rhel-62-64-bit variant[.|https://github.com/mongodb/mongo/blob/74306a6fd07a7194567f77c930e0dc4e18098df3/etc/evergreen.yml#L1552.] As specified in the Design doc, the selected_tests_gen task should run tasks from all required builders.

As a MongoDB engineer,
I should be confident that the selected_tests_gen task is running any affected tasks on all required builders,
so that no required tasks are missed.

AC:

• selected_tests_gen task can result in tasks being run on enterprise-rhel-62-64-bit, enterprise-windows-required, linux-64-debug, enterprise-ubuntu-dynamic-1604-clang, and ubuntu1804-debug-aubsan-lite.