tests: arm thread swap: save and restore callee-saved in subroutine

Temporarily save and restore all callee-saved registers
in the sub-routine, not only the frame pointer, to avoid
errors if compiler chooses to use them in the alternative
thread function. The callee-saved regs are restored after
the alternative thread switches back in.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>