torture: Use correct path for Kconfig fragment for duplicates

Currently, the rcutorture scripting will give an error message if
running a duplicate scenario that happens also to have a non-existent
build directory (b1, b2, ... in the rcutorture directory).  Worse yet, if
the build directory has already been created and used for a real build,
the script will silently grab the wrong Kconfig fragment, which could
cause confusion to the poor sap (me) analyzing old test results.  At
least the actual test runs correctly...

This commit therefore accesses the Kconfig fragment from the results
directory corresponding to the first of the duplicate scenarios, for
which a build was actually carried out.  This prevents both the messages
and at least one form of later confusion.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>