test: use unittest and test_runner for test framework unit testing

Summary:

Test the test_framework, but don't use test_framework objects or functions to test itself

Use python unittest library and put test_framework's unit tests inside their respective files
Add the filename to TEST_FRAMEWORK_MODULES in test_runner
Aggregate all test_framework tests into one TestSuite to run before the functional tests in test_runner
Delete framework_test_script, move test_bn2vch to script.py and add to TEST_FRAMEWORK_MODULES in test_runner

This is a backport of Core PR18576

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9068