Add bn2vch test to functional tests

Summary:
Add a new test for a helper function in the test framework (added in D8852)
Introduce a new prefix for functional tests: framework_test

This concludes backport of Core PR18378
https://github.com/bitcoin/bitcoin/pull/18378/commits/a733ad514a172a77309b84cbc6c81562bdf12e28

Depends on D8852

Test Plan: test/functional/test_runner.py framework_test_script

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8855