torture: Use file-based protocol to mark batch's runs complete

Currently, the script generated by kvm.sh does a "wait" to wait on both
the current batch's guest OSes and any jitter.sh scripts.  This works,
but makes it hard to abstract the jittering so that common code can be
used for both local and distributed runs.  This commit therefore uses
"build.run" files in scenario directories, and these files are removed
after the corresponding scenario's guest OS has completed.

Note that --build-only runs do not create build.run files because they
also do not create guest OSes and do not run any jitter.sh scripts.

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