validate:launcher: Add support for relative path while providing file path

Instead of providing full absolute path while validating the file, should be
able to provide the relative path with respect to the present directory.

https://bugzilla.gnome.org/show_bug.cgi?id=753494

validate/launcher/apps/gstvalidate.py

@@ -683,6 +683,7 @@ not been tested and explicitely activated if you set use --wanted-tests ALL""")
 
             for path in self.options.paths:
                 if os.path.isfile(path):
+                    path = os.path.abspath(path)
                     self._discover_file(path2url(path), path)
                 else:
                     for root, dirs, files in os.walk(path):