Post-push fixes making it work on pushbuild's valgrind host, and clarifying the output.


mysql-test/lib/My/CoreDump.pm:
  - Improved parsing of mtr output so that it works on pushbuild's "valgrind" host.
  - Added stack trace for the thread that coredumped, to make output more readable when there are many threads.
  - Added explanation of what the output consists of.
  - Added early removal of temp file.