kselftest/arm64: Poll less often while waiting for fp-stress children

kselftest/arm64: Poll less often while waiting for fp-stress children

While fp-stress is waiting for children to start it doesn't send any
signals to them so there is no need for it to have as short an epoll()
timeout as it does when the children are all running. We do still want to
have some timeout so that we can log diagnostics about missing children but
this can be relatively large. On emulated platforms the overhead of running
the supervisor process is quite high, especially during the process of
execing the test binaries.

Implement a longer epoll() timeout during the setup phase, using a 5s
timeout while waiting for children and switching  to the signal raise
interval when all the children are started and we start sending signals.

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-v2-2-bd3cef48c22c@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>