dapp-tests: use bash_unit as a test runner

[d-xo]

Description

Makes use of bash_unit as a unit test runner for the dapp_tests. This makes working with these tests a little more ergonomic and means we can:

• esaily run a subset of the tests
• have slightly more readable assertions
• get clearer error messages when an assertion does not hold

If you want to hack on the tests, you can cd into src/dapp-tests and entering the nix-shell. From there you can run bash_unit integeration/tests.sh. To run a subset of the tests, you can run bash_unit -p <REGEX> integration/tests.sh.

I also refactored a little bit, and now the tests are organised so that there is a shared instance of dapp testnet running in the background for every test, as well as a helper function fresh_account that generates and funds a new address.

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

[d-xo]

wrote a mini fuzz testing framework and fuzzed a few easy roundtrip properties. Found a bug in --from-fix, and another in --to-fix.