[ci] allow python test suite to estimate number of jobs.

Summary:
Change build script to not pass number of jobs to test runner. Previously, the
python test runner was updated to estimate the number of jobs to use, while
providing some overhead for the fact that bitcoind uses multiple threads internally.

Test Plan:

Check if this build passes on CI.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1911