software node: add basic tests for property entries

This adds tests for creating software nodes with properties supplied by
PROPERTY_ENTRY_XXX() macros and fetching and validating data from said
nodes/properties.

We are using KUnit framework for the tests.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>