torture: Create a "batches" file for build reuse

This commit creates a "batches" file in the res/$ds directory, where $ds
is the datestamp.  This file contains the batches and the number of CPUs,
for example:

1 TREE03 16
1 SRCU-P 8
2 TREE07 16
2 TREE01 8
3 TREE02 8
3 TREE04 8
3 TREE05 8
4 SRCU-N 4
4 TRACE01 4
4 TRACE02 4
4 RUDE01 2
4 RUDE01.2 2
4 TASKS01 2
4 TASKS03 2
4 SRCU-t 1
4 SRCU-u 1
4 TASKS02 1
4 TINY01 1
5 TINY02 1
5 TREE09 1

The first column is the batch number, the second the scenario number
(possibly suffixed by a repetition number, as in "RUDE01.2"), and the
third is the number of CPUs required by that scenario.  The last line
shows the number of CPUs expected by this batch file, which allows
the run to be re-batched if a different number of CPUs is available.

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