ktest: Add logfile to failure directory

The logfile contains a lot of useful information about the tests being
run. Add it to the stored failure directory when the test fails.

Cc: John 'Warthog9' Hawley <warthog9@kernel.org>
Link: https://patch.msgid.link/20260420142315.7bbc3624@fedora
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>