lib/crypto: tests: polyval_kunit: Increase iterations for preparekey in IRQs

lib/crypto: tests: polyval_kunit: Increase iterations for preparekey in IRQs

On my development machine the generic, memcpy()-only implementation of
polyval_preparekey() is too fast for the IRQ workers to actually fire.
The test fails.

Increase the iterations to make the test more robust.
The test will run for a maximum of one second in any case.

[EB: This failure was already fixed by commit c31f4aa8fed0 ("kunit:
Enforce task execution in {soft,hard}irq contexts").  I'm still applying
this patch too, since the iteration count in this test made its running
time much shorter than the other similar ones.]

Fixes: b3aed551b3fc ("lib/crypto: tests: Add KUnit tests for POLYVAL")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/r/20260102-kunit-polyval-fix-v1-1-5313b5a65f35@linutronix.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>