de8905adf2 test: use unittest and test_runner for test framework unit testing (Gloria Zhao)
Pull request description:
Proposal for unit testing on test_framework functions:
1. Use the python `unittest` library. Don't use test_framework to test itself.
2. Put the tests inside the same file as the functions they are testing.
3. Call the tests from `test_runner.py`. To include more Test Framework tests, add the filename to the list `TEST_FRAMEWORK_MODULES`. Don't add new files or change the list of accepted script prefixes.
Makes these changes for `bn2vch` (followup to [this comment](https://github.com/bitcoin/bitcoin/pull/18378#pullrequestreview-377271264)).
ACKs for top commit:
jnewbery:
Tested ACK de8905adf2. Great stuff gzhao408 . Thanks for this!
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