tests: drivers: flash: Make testcase depend on FLASH_HAS_DRIVER_ENABLED

Change default flash driver testcase to depend on FLASH_HAS_DRIVER_ENABLED,
and the presence of a storage partition in the output DTS, so that the test
will run on all platforms that report flash support.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>