rcutorture: Record results from repeated runs of the same test scenario

Repeatedly running a given test, for example, by repeating the name
as in "--configs "TREE08 TREE08 TREE08" records the results only of
the last run of this test.  This is because the earlier results are
overwritten by the later results.

This commit therefore checks for earlier results, using numbered
file extensions to distinguish multiple runs.  The earlier example
would therefore create directories TREE01, TREE01.2, and TREE01.3.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>