Fix deletion of FinishBreakpoints

Currently, FinishBreakpoints are set at the return address of a frame based on
the `finish' command, and are meant to be temporary breakpoints. However, they
are not being cleaned up after use, as reported in PR python/18655. This was
happening because the disposition of the breakpoint was not being set
correctly.

This commit fixes this issue by correctly setting the disposition in the
post-stop hook of the breakpoint. It also adds a test to ensure this feature
isn't regressed in the future.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18655

gdb/python/py-finishbreakpoint.c

@@ -145,7 +145,7 @@ bpfinishpy_post_stop_hook (struct gdbpy_breakpoint_object *bp_obj)
     {
       /* Can't delete it here, but it will be removed at the next stop.  */
       disable_breakpoint (bp_obj->bp);
-      gdb_assert (bp_obj->bp->disposition == disp_del);
+      bp_obj->bp->disposition = disp_del_at_next_stop;
     }
   catch (const gdb_exception &except)
     {