change the size of core files.

Suppress the 'setrlimit could not change the size of the core files'
warning in mysql-test-run. We do not want core files on some of the
PushBuild hosts, and PushBuild itself does not set --core-files, so
that warning is expected.


mysql-test/lib/mtr_report.pl:
  Suppress the 'setrlimit could not change the size of the core files'
  warning in mysql-test-run. We do not want core files on some of the
  PushBuild machines, and PushBuild itself does not set --core-files, so
  that warning is expected.