Fix test expectation

As of PHP 8.0.0, array to string conversion raises a warning.

Author: cmb69

ext/mbstring/tests/bug79149.phpt

@@ -13,9 +13,9 @@ var_dump(mb_convert_encoding('foo', 'UTF-8', array(['bar'], ['baz'])));
 Warning: mb_convert_encoding(): Illegal character encoding specified in %s on line %d
 string(0) ""
 
-Notice: Array to string conversion in %s on line %d
+Warning: Array to string conversion in %s on line %d
 
-Notice: Array to string conversion in %s on line %d
+Warning: Array to string conversion in %s on line %d
 
 Warning: mb_convert_encoding(): Illegal character encoding specified in %s on line %d
 string(3) "foo"