torture: Extract testid.txt generation to separate script

torture: Extract testid.txt generation to separate script

The kvm.sh script places a testid.txt file in the top-level results
directory in order to identify the tree and commit that was tested.
This works well, but there are scripts other than kvm.sh that also create
results directories, and it would be good for them to also identify
exactly what was tested.

This commit therefore extracts the testid.txt generation to a new
mktestid.sh script so that it can be easily used elsewhere.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>