[CI] Improve performance when running the tests

Summary:
This diff removes the explicit call to bitcoin-util-test.py which is
already run by make check.
It also adds the -j option to make check. Tested on my machine this
resulted in a ~25% average time improvement.

Test Plan:

./build.sh # builds and runs tests

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2753