kselftest/arm64: Flag fp-stress as exiting when we begin finishing up

Once we have started exiting the termination handler will have the same
effect as what we're already running so set the termination flag at that
point.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220921181345.618085-3-broonie@kernel.org


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