gdb: add selftest name completion

After the previous commit, it is easy to add completion for selftest
names.  Again, this is not particularly high value, but I rarely touched
completion, so it served as a simple example to get some practice.

Change the for_each_selftest_ftype parameter to gdb::function_view, so
that we can pass a lambda that captures things.

Change-Id: I87cac299ddca9ca7eb0ffab78342e850a98d954c