test/blobfs: Refactor exit behavior

The on_error_exit() function was being called via the local ERR trap
to cleanup after the tests in case any failure occurred during the
execution. However, this was masking the backtrace trap so the
on_error_exit() had to also call print_backtrace() to compensate -
this was resulting in an additional function being unnecessarily
placed onto the stack and included in the actual backtrace.

Avoid the above by refactoring on_error_exit() into simple cleanup()
function called upon exit()ing from the script. The rationale is that
cleanup has to happen regardless if the script failed or not and
elements like $conf_file are always created from the very scratch by
the script itself anyway prior running the tests.

Change-Id: I6bb8f4155525037624f0bd5aab288f8c502141f6
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/456


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>