kselftest/arm64: Cover ZT in the FP stress test

Hook up the newly added zt-test program in the FPSIMD stress tests, start
a copy per CPU when SME2 is supported.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-15-f2fa0aef982f@kernel.org


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