fuzz: Pass down MAKEJOBS to test_runner

Summary:
Backport of core PR18929.

Depends on D8277.

Test Plan:

ninja bitcoin-fuzzers
./test/fuzz/test_runner.py -j 20 <path_to_corpus>

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8278