[tx] Test collecting tempids after upsert resolution

[ncalexan]

In #283, I didn't quite finish the tempid collection implementation. It's difficult to test without #294 since there aren't attributes with the correct form in the bootstrap database.

This ticket tracks actually testing the expected-tempids in files like

mentat/tx/fixtures/test_upsert_vector.edn

Line 29 in 16e9740

{"t1" 100

.

This will get better after #271.