sched_ext: Add option -l in selftest runner to list all available tests

sched_ext: Add option -l in selftest runner to list all available tests

The selftest runner currently allows selecting tests via the -t
option. This patch adds a new -l option that lists all available tests,
providing users with an overview of the tests they can choose from. This
enhancement is especially useful for scripting and automation purposes,
making it easier to discover and run tests.

Signed-off-by: Shizhao Chen <shichen@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>