ci: Better core dump checking

- Do not exclude any directories from `find` search, remove dumps before tests
  instead.
- Install `apport` on travis so that linux tests should produce core dumps
  (based on information from travis-ci/travis-ci#3754, not sure whether it still
  applies).
- Check cores in lua so that one has an idea which test is failing exactly. Do
  this only 10% of time on linux because traversing the file system is slow.

Unit tests are still not touched, though it is what `app` argument in
`check_cores` is for.

TODO? consider using `find`, it may be faster. Consider retiring `os.execute`,
      dealing with escaping is bad.

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