Slurm environment variables can modify the behaviour of job submission implicitly

[vkarak]

Slurm envirionment variables, e.g. SBATCH_CONSTRAINT, SBATCH_PARTITION, can modify the behaviour of test job submissions implicitly bypassing the configuration and the -J command-line option. The job submission should be fully controlled by ReFrame, so ReFrame should be unsetting all SBATCH_* environment variables before submitting.

There is the sched_access_in_submit configuration option introduced in #3156 that will work around this, but imo this is still a limitation and the default behaviour should not be dependency on Slurm environment variables.

@boegel @casparvl @laraPPr I think such behaviour would avoid the issue #2970 altogether, since iirc you were using SBATCH_PARTITION and ReFrame could not override this, so you had to rely on the new option introduced in #3156.