tests: Use CALLS_PLUGIN_DIR to search for plugins in the build dir

There is no need to add a second variable to search for plugins, we can
use CALLS_PLUGIN_DIR directly.

plugins/provider/tests/test-sip.c

@@ -429,10 +429,6 @@ main (int   argc,
   gtk_test_init (&argc, &argv, NULL);
 
   gst_init (NULL, NULL);
-
-#ifdef PLUGIN_BUILDDIR
-  peas_engine_add_search_path (peas_engine_get_default (), PLUGIN_BUILDDIR, NULL);
-#endif
   /* this is a workaround for an issue with sofia: https://github.com/freeswitch/sofia-sip/issues/58 */
   su_random64 ();