ktest.pl: Define PRE_TEST_DIE to kill the test if the PRE_TEST fails

Currently, if a PRE_TEST is defined and ran, but fails, there's nothing
currently available to make the test fail too. Add a PRE_TEST_DIE option that
when set, if a PRE_TEST is defined and fails, the test will die too.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>