torture: Make kvm-find-errors.sh notice missing vmlinux file

torture: Make kvm-find-errors.sh notice missing vmlinux file

Currently, an obtuse compiler diagnostic can fool kvm-find-errors.sh
into believing that the build was successful.  This commit therefore
adds a check for a missing vmlinux file.  Note that in the case of
repeated torture-test scenarios ("--configs '2*TREE01'"), the vmlinux
file will only be present in the first directory, that is, in TREE01
but not TREE01.2.

Link: https://lore.kernel.org/lkml/36bd91e4-8eda-5677-7fde-40295932a640@molgen.mpg.de/
Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>