kselftest/arm64: Increase frequency of signal delivery in fp-stress

Currently we only deliver signals to the processes being tested about once
a second, meaning that the signal code paths are subject to relatively
little stress. Increase this frequency substantially to 25ms intervals,
along with some minor refactoring to make this more readily tuneable and
maintain the 1s logging interval. This interval was chosen based on some
experimentation with emulated platforms to avoid causing so much extra load
that the test starts to run into the 45s limit for selftests or generally
completely disconnect the timeout numbers from the

We could increase this if we moved the signal generation out of the main
supervisor thread, though we should also consider that he percentage of
time that we spend interacting with the floating point state is also a
consideration.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241030-arm64-fp-stress-interval-v2-1-bd3cef48c22c@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>