test: drivers: Move existing flash tests to flash/common directory

After 'flash_ex_op' syscall was added we are able to expose vendor
specific features to the user.

This patch moves existing tests to 'tests/drivers/common'. New tests for
vendor specific operations should go to vendor directory under
'tests/drivers/flash'.

Signed-off-by: Patryk Duda <[email protected]>

Author: duda-patryk