of: unittest: for strings, account for trailing \0 in property length field

For strings, account for trailing \0 in property length field:

This is consistent with how dtc builds string properties.

Function __of_prop_dup() would misbehave on such properties as it duplicates
properties based on the property length field creating new string values
without trailing \0s.

Signed-off-by: Stefan M Schaeckeler <sschaeck@cisco.com>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>