torture: Move build/run synchronization files into scenario directories

Currently the bN.ready and bN.wait files are placed in the
rcutorture directory, which really is not at all a good place
for run-specific files.  This commit therefore renames these
files to build.ready and build.wait and then moves them into the
scenario directories within the "res" directory, for example, into
tools/testing/selftests/rcutorture/res/2021.02.10-15.08.23/TINY01.

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