Separate smoke tests from stress tests in Evergreen

Modify our Evergreen test configuration to separate the smoke tests (i.e., format -S) from the stress tests.  WT-8321 changed Evergreen to run these tests just once for each relevant build variant. So it no longer makes sense to include them with the long-running stress tests.

luke.chen suggests:

It makes sense to move the smoke tasks out of the stress testing group after they are reduced to running only once. !Ubuntu 20.04 and !Ubuntu 20.04 ASAN build variants could be their new homes. I think we can keep them out of the PR testing scope (i.e. not tagging pull_request) for now.