libweston: drop opaque struct wet_testsuite_data

Until now we had struct wet_testsuite_data as an opaque
struct that should be defined by the testsuite of libweston
users. Instead, keep the data as a void * and document that
users are responsible for defining the data type.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

include/libweston/libweston.h

@@ -203,7 +203,7 @@ struct weston_testsuite_quirks {
  */
 struct weston_testsuite_data {
 	struct weston_testsuite_quirks test_quirks;
-	struct wet_testsuite_data *test_private_data;
+	void *test_private_data;
 };
 
 /** Represents a head, usually a display connector