of: unittest: Add tests for changeset properties adding

No test cases are present to test the of_changes_add_prop_*() function
family.

Add a new test to fill this lack.
Functions tested are:
  - of_changes_add_prop_string()
  - of_changes_add_prop_string_array()
  - of_changeset_add_prop_u32()
  - of_changeset_add_prop_u32_array()

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20240527161450.326615-15-herve.codina@bootlin.com


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>