clk: tests: Add some tests for orphan with multiple parents

Let's leverage the dummy mux with multiple parents we have to create a
mux whose default parent will never be registered, and thus will always
be orphan by default.

We can then create some tests to make sure that the clock API behaves
properly in such a case, and that the transition to a non-orphan clock
when we change the parent is done properly.

Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com> # imx8mp
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com> # exynos4210, meson g12b
Signed-off-by: Maxime Ripard <maxime@cerno.tech>