selftests/user_events: Wait for deferred event teardown after unregister

Unregistering a user event now defers the drop of the enabler's event
reference (and the freeing of the enabler) past an RCU grace period. As a
result DIAG_IOCSDEL can transiently fail with -EBUSY while that last
reference is still being dropped, where it previously succeeded
immediately.

Two tests assumed the delete takes effect the instant the unregister
returns:

  - abi_test "flags" deletes the event right after disabling it.
  - perf_test's fixture teardown clear() deletes __test_event before the
    next test registers the same name; a stale event makes the following
    registration fail with -EADDRINUSE.

Retry the delete until it succeeds (or the event is already gone) with a
bounded wait, matching the existing wait_for_delete() idiom in the same
suite, so the tests are robust to the deferred teardown.

Link: https://patch.msgid.link/20260707180240.2887081-1-michael.bommarito@gmail.com
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

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