[gdb/testsuite] Handle auto_left_margin in tuiterm

[gdb/testsuite] Handle auto_left_margin in tuiterm

The terminal capability bw (aka as auto_left_margin) controls whether a
backspace at the start of a line wraps to the last column of the previous
line.

For tuiterm, we use TERM=ansi, and on linux at least that capability is off.

Consequently the current implementation of Term::_ctl_0x08 doesn't wrap.

Add this capability in Term::_ctl_0x08, and add a unit test.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>