kunit: irq: Continue increasing hrtimer interval for longer

Currently, kunit_irq_test_timer_func() stops increasing the hrtimer
interval as soon as some forward progress is made in each of softirq and
task context.  Update it to use a more aggressive strategy: increase the
interval as long as the hrtimer is running significantly faster than
either context.

This resolves an occasional hang in the CRC and crypto library tests
under qemu-system-s390x.  It was exposed by the change in the default
preemption model on s390 from NONE to LAZY.  That seems to have exposed
the issue by allowing some forward progress to be made while the actual
system timer tick is still starved, preventing jiffies from increasing
or the task context from making much progress towards max_iterations.

Fixes: 201ceb94aa1d ("kunit: irq: Ensure timer doesn't fire too frequently")
Cc: stable@vger.kernel.org
Reviewed-by: David Gow <david@davidgow.net>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://patch.msgid.link/20260803181842.44648-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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