native_types: add != comparison for string_wrapper

If a 'type: path' is added to a sdbus++ interface YAML file, the
resulting code does not compile due to a missing != comparison
operator for string_wrapper.  Add it and enhance the test case to
cover.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iaa661a15e58a99e4e816c391152926e9af1a8339
diff --git a/test/server/Test.interface.yaml b/test/server/Test.interface.yaml
index 2f86d70..2b6c9f5 100644
--- a/test/server/Test.interface.yaml
+++ b/test/server/Test.interface.yaml
@@ -14,3 +14,5 @@
           - const
     - name: Countable
       type: size
+    - name: ObjectPath
+      type: path