kselftest/arm64: Don't drain output while spawning children

Now we hold execution of the stress test programs until all children are
started there is no need to drain output while that is happening.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221129215926.442895-3-broonie@kernel.org


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