torture: Rename SRCU-t and SRCU-u to avoid lowercase characters

The convention that scenario names are all uppercase has two exceptions,
SRCU-t and SRCU-u.  This commit therefore renames them to SRCU-T and
SRCU-U, respectively, to bring them in line with this convention.  This in
turn permits tighter argument checking in the torture-test scripting.

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