selftests/sched_ext: Cover duplicate DSQ creation and ID reuse

The create_dsq test creates and destroys queues, but does not check the
-EEXIST contract for duplicate creation or whether the ID remains
accessible after the failed operation.

Extend the existing lifecycle loop to reject duplicate creation and
check that the empty queue is still accessible. After destruction,
require -ENOENT from the queue lookup, then recreate and destroy the
same ID. Check that all 1024 iterations complete.

This exercises empty queues during initialization, without concurrent
enqueueing or destruction of a nonempty queue. The create_dsq test
passes on a two-CPU matching-kernel VM.

Link: https://lore.kernel.org/all/Z-OZ7tJWhRZbUk1l@gpd3/
Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
Signed-off-by: Tejun Heo <tj@kernel.org>

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