tests/subsys: CONFIG_TEST_FLASH_DRIVERS=y only for qemu_x86

CONFIG_TEST_FLASH_DRIVERS is dedicated for qemu_x86.
This patch moves it to per board project configuration.

Setting this property for each target will cause build
failures.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>