tests: clock_control: stm32h7_devices: Fix clock source check

Test was using "clock-names" property to query domain clock configuration.
This is not working since clock-names was removed in the last step of the
feature implementation and whole macro was always reporting DT_NO_CLOCK.

This issue went undetected because of an additional issue in the exception
case which was testing "zassert_true(1, .." instead of "zassert_true(0, .".

Fix both issues to make the test efficient again.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>