[gdb/testsuite] Fix Cursor Horizontal Absolute clipping

I looked at the tuiterm implementation of Cursor Horizontal Absolute:
...
    proc _csi_G {args} {
	set arg [_default [lindex $args 0] 1]

	_log_cur "Cursor Horizontal Absolute ($arg)" {
	    variable _cur_col
	    variable _cols

	    set _cur_col [expr {min ($arg - 1, $_cols)}]
	}
    }
...
and noticed a problem with the clipping behavior.

If we have say $_cols == 80, and we do _csi_G 81 we get $_cur_col == 80, while
$_cur_col is zero-based and should be in the 0..79 range.

Fix this by using:
...
	    set _cur_col [expr {min ($arg, $_cols)} - 1]
...
which gets us $_cur_col == 79.

Add two boundary tests to gdb.tui/tuiterm.exp.

Tested on x86_64-linux.

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