torture: Make torture.sh create a properly formatted log file

Currently, if the torture.sh allmodconfig step fails, this is counted as
an error (as it should be), but there is also an extraneous complaint
about a missing log file.  This commit therefore adds that log file,
which is hoped to reduce confused reactions to the error report.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>