tests: drivers: dac loopback testcase for nucleo g071rb

Running the testcase dac loopback on the nucleo_g071rb
target board.
The ADC1 in0 on PA0 (pin A0), the DAC1 out1 on PA4 (pin A2)
must be connected on the HW.

Signed-off-by: Francois Ramu <francois.ramu@st.com>