LU-12760 tests: stack_trap defaults to sigspec=EXIT

LU-12760 tests: stack_trap defaults to sigspec=EXIT

If the "sigspec" argument is not specified for stack_trap(), default
to "EXIT" as the signal, since this is what we use for all callers
of stack_trap() today anyway.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I2c8d986cdf8743e1d956cd7941a47bd4cd772592
Reviewed-on: https://review.whamcloud.com/36186
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>