test: use a regexp in ecash-agora test

Summary:
The error message is now more specific, it mentions the txid and input index of the failing tx as well as the outpoint being spent.
So match a regex instead of trying to get the exact error message with potentially non-deterministic data in it.

Test Plan: ecash-agora-integration-tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18946