[petsc-dev] CI update: valgrind test mandatory

[Balay, Satish]

All petsc Merge Request creators,

The issues with valgrind test are now fixed - so this test no longer
has 'allow_failure' flag set.

So its best to rebase all MRs over latest master [or maint - if
appropriate] - before submitting test pipelines - this way - we are
using the valgrind test results correctly [and not inadvertently
merging in MRs with valgrind test failures]

Thanks,
Satish