Avoid duplication in test/format failure CONFIG set

WT-5645 introduced a new Evergreen task cycling through a set of test/format CONFIGs (under test/format/failure_configs) that failed previously and resolved by certain WT tickets. If one of those CONFIGs were failed again in a newer testing run (indicating a new issue or breakage captured) with a new ticket created, we'd like to avoid the same CONFIG being added into the failure CONFIG set and causing test coverage duplication. 

The current naming convention for the set of failure CONFIGs is CONFIG.WT-xxxx.