tests/kernel: Avoid C library calls from interrupt handlers

On arm64, GCC feels free to use floating point registers for essentially
anything unless we build with -mgeneral-regs-only. After the FPU gets used
in an interrupt handler, interrupts are disabled as there's no place to
save the registers during a nested exception. As the C library may be built
separately without this flag, we cannot use any of its functions or risk
having nested exceptions fail.

Switch printk usage to k_str_out and stick to (mostly) Zephyr internal
functions to ensure nested interrupts work correctly.

Signed-off-by: Keith Packard <keithp@keithp.com>