tests: arm_thread_swap_tz: Enhance testing of FP regs

The existing test puts the same value into all FP registers, runs a
secure service which is then interrupted by a thread that clobbers the
registers.

This patch instead puts different values into each register to test
that each register is restored in the right order.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>