drivers/dac: stm32: Enabling DAC tests for stm32f3_disco

This commit enables test_dac to build and run on stm32f3_disco
platform.
This has been tested with twister and also as a standalone test_dac
test and is working as expected.

Signed-off-by: Krishna Mohan Dani <krishnamohan.d@hcl.com>