[tx] Use better keywords in transaction testing

[ncalexan]

This ticket tracks replacing :expecteddatoms and :expectedtransactions around

mentat/db/src/db.rs

Line 863 in 4258053

let expected_transaction: Option<&edn::Value> = transaction.get(&edn::Value::NamespacedKeyword(symbols::NamespacedKeyword::new("test", "expectedtransaction")));

with :expected-datoms and :expected-transactions; and updating the Wiki documentation at https://github.com/mozilla/mentat/wiki/Transacting:-EDN-test-format to reflect the changes.

For context, in #214, I landed the first parts of the transaction processor, and I landed the first parts of #188, which starts converting tests from the Clojure implementation to a generic EDN format. While implementing the code in db.rs linked above, I ran into #219, which is now fixed. This ticket tracks patching up the provisional names above.

[ncalexan]

Oh -- if there are other :multiword keywords in that code when you get to this, please make them all :multi-word. (That's EDN style.)

Also, the test files like fixtures/*.edn will need to be updated as well.