tests/drivers/clock_control: stm32_common_devices: add adc alt. clk. src

This commit adds a test case that configures an alternative clock source
for an ADC peripheral.

In case no alt clock is available, only the gating clock is enabled
and disabled.
Unlike the i2c and lptim test, the actual gating clock frequency is
not checked, because for the adc there seems to be no uniform way
to retrieve the frequency via the hal.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>