torture: Improve readability of the testid.txt file

The testid.txt file was intended for occasional in extremis use, but
now that the new "bare-metal" file references it, it might see more use.
This commit therefore labels sections of output and adds spacing to make
it easier to see what needs to be done to make a bare-metal build tree
match an rcutorture build tree.

Of course, you can avoid this whole issue by building your bare-metal
kernel in the same directory in which you ran rcutorture, but that might
not always be an option.

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