of: Add a KUnit test for overlays and test managed APIs

Test the KUnit test managed overlay APIs. Confirm that platform devices
are created and destroyed properly. This provides us confidence that the
test managed APIs work correctly and can be relied upon to provide tests
with fake platform devices and device nodes via overlays compiled into
the kernel image.

Cc: Rob Herring <robh@kernel.org>
Cc: Saravana Kannan <saravanak@google.com>
Cc: Daniel Latypov <dlatypov@google.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Reviewed-by: David Gow <davidgow@google.com>
Cc: Rae Moar <rmoar@google.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240718210513.3801024-5-sboyd@kernel.org