Add tests that backtrace and backtrace_symbols produce correct results

[David Miller]

From: Andreas Schwab <schwab@linux-m68k.org>
Date: Sun, 20 Jan 2013 00:26:51 +0100

> You cannot expect reliable backtrace through a signal trampoline without
> unwind information.

It would be simple to add a signal frame sniffer to the
non-unwind-information paths of the x86 backtrace code.

Both the unwinder in libgcc and gdb can do this, glibc
thus can too.

I plan to add such code to the space backtracer.