CI: attempt to split loop to speed up valgrind build
Currently vangrind [and perhaps other jobs] take a long time due to sequential nature of test harness [esp loops and perhaps test set].
It would be great if test harness could automatically parallelize such jobs. Until then - this MR should improve CI times.
before change: 191 minutes
https://gitlab.com/petsc/petsc/-/jobs/1123151904
after change: 117 minutes
https://gitlab.com/petsc/petsc/-/jobs/1122775562
Edited by Satish Balay