rcutorture: Simplify logging

Both the 'kvm.sh' and 'kvm-test-1-run.sh' scripts log messages by printing
the message to 'stdout' and then also printing it into the log file.
Generation of the message thus occurs twice, once for 'stdout' and once
for the log file.  Moreover, many of the messages contain 'date' output,
which results in date being invoked twice (once for stdout print, once
for log file write).  As a result, the date information in stdout and
log file can differ, which could cause confusion.

This commit therefore simplifies the logging procedure by using 'tee'.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>