[gdb/testsuite] Handle Horizontal Position Absolute in tuiterm

[gdb/testsuite] Handle Horizontal Position Absolute in tuiterm

I ran the tui testsuite on freebsd with TERM=ansiw, and investigated the first
failure, in test-case gdb.tui/tui-init-source.exp.

The problem turned out to be the lack of handling a Horizontal Position
Absolute [1] sequence "^[[80`" in tuiterm.

Add Term::_csi_`, forwarding to Term::_csi_G which handles Cursor Horizontal
Absolute [2].

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://vt100.net/docs/vt510-rm/HPA.html
[2] https://vt100.net/docs/vt510-rm/CHA.html