Fix "set debug timestamp"

PR cli/29945 points out that "set debug timestamp 1" stopped working
-- this is a regression due to commit b8043d27 ("Remove a ui-related
memory leak").

This patch fixes the bug and adds a regression test.

I think this should probably be backported to the gdb 13 branch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29945