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

[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.