testutil: quickfix-compatible stack trace format
testutil: quickfix-compatible stack trace format

This makes test stack traces a little more readable, and
compatible with the common "quickfix" format, supported by
some text editors.

Closes chain/chainprv#910.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain