tests/kernel/sched/deadline: Add a SCHED_SCALABLE=y case for coverage

It turns out the deadline scheduling and the scalable scheduling
backend didn't work right together (for almost six years!) because we
never had coverage of that combination.  Add it explicitly given the
known issue in #69935.

We might want to consider adding a whole CI target with SCALABLE=y,
which we've done for other common tunables to get coverage.

Signed-off-by: Andy Ross <andyross@google.com>