torture: Create kvm-check-branches.sh output in proper location

Currently, kvm-check-branches.sh causes each kvm.sh invocation create a
separate date-stamped directory, then after that invocation completes,
moves it into the *-group/NNNN directory.  This works, but makes it more
difficult to monitor an ongoing run.  This commit therefore uses the
kvm.sh --datestamp argument to make kvm.sh put the output in the right
place to start with, and also dispenses with the additional level of
datestamping.  (Those wanting datestamps can find them in the log files.)

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