test: replace hand-rolled backtrace function with gstack

Let's use something that specializes in that task and does a better job of it than whatever we'll come up with. Due to how it's implemented the stacktrace will always show waitpid() as frame 0 now but we can live with that.

gstack prints to stdout but litest_log() uses stderr, so we cannot just call system(), we have do do the pipe/fork/exec/waitpid/read dance. We could use that to filter the #0 frame showing waidpid() from gstack but meh.

This drops the libunwind and addr2line dependency and replaces it with gstack instead.