Add check for existing output files during validation

[nweires]

This is to avoid the problem of output files from a previous job being mixed with those from a later job that writes to the same location.

Currently, files can end up mixed together if either 1) the second job had few tasks (so didn't overwrite all the existing files) or 2) the second job had some tasks fail.

[github-actions]

File	Coverage
All files	86%	✅
base.py	91%	✅
exc.py	57%	✅
hpc.py	78%	✅
local.py	70%	✅
postprocessing.py	84%	✅
utils.py	91%	✅
cloud/docker_base.py	79%	✅
sampler/base.py	79%	✅
sampler/downselect.py	33%	✅
sampler/precomputed.py	93%	✅
sampler/residential_quota.py	61%	✅
test/shared_testing_stuff.py	85%	✅
test/test_docker.py	33%	✅
test/test_local.py	97%	✅
test/test_validation.py	97%	✅
workflow_generator/base.py	90%	✅
workflow_generator/commercial.py	53%	✅
workflow_generator/residential_hpxml.py	86%	✅

Minimum allowed coverage is 33%

Generated by 🐒 cobertura-action against 400e94f

[mfathollahzadeh]

Thanks! Looks good to me!