Fullstack job is using different tox env in the gate
(dsvm-fullstack-gate). To reduce memory usage in that job, some
time ago, in patch  we reduced number of test workers from 4
to 3. But it was done for "dsvm-fullstack" tox env, not for
"dsvm-fullstack-gate" thus it had no effect on our CI runs.
Now it is changed also in the CI job's tox env.
(cherry picked from commit