Increment transaction index in TransactionInfo

[mikedotexe]

Currently the transaction index is hardcoded to zero:

cw-multi-test/src/wasm.rs

Line 838 in 42d8e9b

transaction: Some(TransactionInfo { index: 0 }),

And I believe we're approaching a time where folks will want to utilize the TransactionInfo more and more, and test that it's incrementing as expected.

[DariuszDepta]

Needs more (re)design in MultiTest, and is not of the highest priority, so I am moving this issue to backlog.