tests: spi: loopback: use CONFIG_ZTEST_STACK_SIZE for thread stacks

The spi_loopback test suite creates three internal threads. The
stack sizes for these threads is hardcoded to 512, which is to little
for some socs, namely the nrf54h20 cpuapp if pm device runtime is
enabled. Instead, determine the stack sizes the same way ztest
stack sizes are determined.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>