[test] add typehints to create_block and create_coinbase

Summary:
These functions are hard to use without first looking at other tests, because the parameter types are not obvious. This improves the documentation and IDE hints for these functions.

Also remove a pair of redundant parentheses.

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11320