Crashing tests with huge backtraces (e.g. unlimited stack recursion) are slow and do not make it into the gerrit failure reports

When parsing a test failure, parse_build_log.pl gives up if the test's output is very large (because we do not want to post multi-megabyte messages into gerrit).

This can be triggered by a test crashing with a stack overflow due to unbounded recursion, as was seen on http://codereview.qt-project.org/7492 .

Also, gdb itself can take a very long time printing the backtrace.

We need to make some kind of limit (on both time and output size) when getting a backtrace.