Add regression test for append-mode negative position fix

Author: lacatoire

ext/standard/tests/streams/gh21700_user_stream_append_negative_position.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-21700: Append-mode open ignores a negative position returned by a user stream
+--FILE--
+<?php
+class lyingstream {
+    public function stream_open($path, $mode, $options, &$opened_path) { return true; }
+    public function stream_seek($offset, $whence) { return true; }
+    public function stream_tell(): int { return -42; }
+    public function stream_eof() { return false; }
+    public function stream_read($count) { return ''; }
+}
+
+stream_wrapper_register("test", "lyingstream");
+
+// Opening in append mode triggers an internal SEEK_CUR to revise the initial
+// position; if the user stream lies, ftell() must still report a non-negative
+// value and a subsequent SEEK_CUR must not trip the position invariant.
+$fp = fopen("test://foo", "ab");
+var_dump(ftell($fp));
+var_dump(fseek($fp, 0, SEEK_CUR));
+fclose($fp);
+
+stream_wrapper_unregister("test");
+?>
+--EXPECT--
+int(0)
+int(-1)