ktest: Store failure logs also in fatal paths

STORE_FAILURES was only saved from fail(), so paths that reached dodie()
could exit without preserving failure logs.

That includes fatal hook paths such as:

  POST_BUILD_DIE = 1

and ordinary failures when:

  DIE_ON_FAILURE = 1

Call save_logs("fail", ...) from dodie() too so fatal failures keep the
same STORE_FAILURES artifacts as non-fatal fail() paths.

Cc: John Hawley <warthog9@eaglescrag.net>
Link: https://patch.msgid.link/20260318-ktest-fixes-v1-1-9dd94d46d84c@suse.com
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>