test: driver_test follows the package-target -o wording

Straggler consumer of the -c -o contract commit; the row's inputs
(-o on a tree target) now take the package-target reject.

test/tool/driver_test.ww

@@ -116,7 +116,7 @@ fn rundrv(root: str, name: str, drv: str, argv: []str,
 		"ww test: unknown flag",
 		"ww test: -I needs an argument",
 		"ww test: -o needs an argument",
-		"ww test: -c/-S/-o need a single test file",
+		"ww test: -o needs -c for a package target",
 		"ww test: -run needs an argument",
 		"ww test: pattern needs a single test file"];
 	let i: i32 = 0;