Tests: Add test for getdifficulty

Summary:
Test added to blockchain.py as adding a new test to reduce test run time.

Backport core's PR10229

Test Plan:

./test/functional/test_runner.py blockchain.py

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Differential Revision: https://reviews.bitcoinabc.org/D951