tests: drivers: spi: allow to configure thread stack size

Used to perform coverage analysis when testing on HW.
Gathering coverage data require more stack,
so its needs to configurable.
There is used CONFIG_TEST_EXTRA_STACK_SIZE for that.
(with value depending on used HW).

Signed-off-by: Piotr Kosycarz <piotr.kosycarz@nordicsemi.no>