Update the TEST_PARALLEL_JOBS value in CI

Update the TEST_PARALLEL_JOBS value in CI

The tests have been executed with four jobs for a long time because the
TEST_PARALLEL_JOBS variable was set to 4 in the project's CI/CD
Variables Settings. The variable was recently dropped in favor of
.gitlab-ci.yaml one and now needs to be streamlined with the rest of
BIND 9 maintained branches.

(cherry picked from commit 03d7b45d81268708fd47bebc8d13f8bc5c49f64b)
(cherry picked from commit 6a975344a2423d4eaf383e90456391bd72cab817)