testrunner: make `-j100 --runs-per-test=100 test-foo.t` use 100 jobs

Authored by martinvonz.

Description

testrunner: make -j100 --runs-per-test=100 test-foo.t use 100 jobs

It would use only 1 job since 1039404c5e1d (run-tests: print number of
tests and parallel process count, 2018-10-13), but I'm pretty sure
that was unintentional.

Differential Revision: https://phab.mercurial-scm.org/D5557