Currently, tests for transactions are mostly marked as WiredTiger only. For example, do_txn_basic.js, multi_statement_transaction_simple.js.

However, it is wrong to specifically skip all storage engines that are
not wiredTiger, as that means that we will not test transactions on inMemory and
encrypted storage engines, even if they do support transactions, or if they
start supporting them in the future. Instead we should have a tag that reflects
whether transactions are supported or not.

This way we only have to adjust a single location if we add/remove support for
transactions for a specific storage engine.