fix ns_plugin_expandpath tests with no extension

fix ns_plugin_expandpath tests with no extension

Parts of ns_plugin_expandpath() test expected the plugin extension to be
appened automatically (the plugin name/path is provided without the
extension), this enable to test the logic which adds the correct
extension based on the platfrom.

But the expected expanded paths from the test were hard coded with the
`.so` extension, so the test can't pass on macOS platform. This fixes
the test by using the macro providing the current-platform extension.