kselftest/arm64: Provide progress messages when signalling children

Especially when the test is configured to run for a longer time it can be
reassuring to users to see that the supervising program is running OK so
provide a message every second when the output timer expires.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221017144553.773176-3-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>