[gdb/testsuite] test a function call by hand from pretty printer

The test case added here is testing the bug gdb/28856, where calling a
function by hand from a pretty printer makes GDB crash. There are 6
mechanisms to trigger this crash in the current test, using the commands
backtrace, up, down, finish, step and continue. Since the failure happens
because of use-after-free (more details below) the tests will always
have a chance of passing through sheer luck, but anecdotally they seem
to fail all of the time.

The reason GDB is crashing is a use-after-free problem. The above
mentioned functions save a pointer to the current frame's information,
then calls the pretty printer, and uses the saved pointer for different
reasons, depending on the function. The issue happens because
call_function_by_hand needs to reset the obstack to get the current
frame, invalidating the saved pointer.

gdb/testsuite/gdb.python/pretty-print-call-by-hand.c

@@ -0,0 +1,53 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see  <http://www.gnu.org/licenses/>.  */
+
+struct mytype
+{
+  char *x;
+};
+
+void
+rec (int i)
+{
+  if (i <= 0)
+    return;
+  rec (i-1);
+}
+
+int
+f ()
+{
+  rec (100);
+  return 2;
+}
+
+void
+g (struct mytype mt, int depth)
+{
+  if (depth <= 0)
+    return; /* TAG: final frame */
+  g (mt, depth - 1); /* TAG: first frame */
+}
+
+int
+main ()
+{
+  struct mytype mt;
+  mt.x = "hello world";
+  g (mt, 10); /* TAG: outside the frame */
+  return 0;
+}