ReFrame does not warn or give an error if a test file does not exist

[vkarak]

It will simply ignore it without any warning:

./bin/reframe -c /foo.py -l

[ReFrame Setup]
  version:           3.6.0-dev.1+ff33e201
  command:           './bin/reframe -c /foo.py -l'
  launched by:       karakasv@dom101
  working directory: '/users/karakasv/Devel/reframe'
  settings file:     '<builtin>'
  check search path: '/foo.py'
  stage directory:   '/users/karakasv/Devel/reframe/stage'
  output directory:  '/users/karakasv/Devel/reframe/output'

[List of matched checks]

Found 0 check(s)

Log file(s) saved in: '/tmp/rfm-x4c_6wmq.log'```