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.

test/functional/helpers.lua

@@ -9,6 +9,7 @@ local TcpStream = require('nvim.tcp_stream')
 local SocketStream = require('nvim.socket_stream')
 local ChildProcessStream = require('nvim.child_process_stream')
 
+local check_cores = global_helpers.check_cores
 local check_logs = global_helpers.check_logs
 local neq = global_helpers.neq
 local eq = global_helpers.eq
@@ -608,7 +609,10 @@ local M = {
 
 return function(after_each)
   if after_each then
-    after_each(check_logs)
+    after_each(function()
+      check_logs()
+      check_cores('build/bin/nvim')
+    end)
   end
   return M
 end