gdb/testsuite: fix line feed scrolling in tuiterm.exp

In a following commit I managed to trigger the line feed scrolling
case in tuiterm.exp.  This case is currently unhandled, and this
commit fills in this missing functionality.

The implementation is pretty simple, just scroll all the content up
one line at a time until the cursor is back on the screen (a single
line of scroll is all that should be needed).

This change is untested in this commit, but is required by the next
commit.

gdb/testsuite/lib/tuiterm.exp

@@ -99,10 +99,24 @@ namespace eval Term {
 	_log_cur "Line feed" {
 	    variable _cur_row
 	    variable _rows
+	    variable _cols
+	    variable _chars
 
 	    incr _cur_row 1
-	    if {$_cur_row >= $_rows} {
-		error "FIXME scroll"
+	    while {$_cur_row >= $_rows} {
+		# Scroll the display contents.  We scroll one line at
+		# a time here; as _cur_row was only increased by one,
+		# a single line scroll should be enough to put the
+		# cursor back on the screen.  But we wrap the
+		# scrolling inside a while loop just to be on the safe
+		# side.
+		for {set y 0} {$y < [expr $_rows - 1]} {incr y} {
+		    set next_y [expr $y + 1]
+		    for {set x 0} {$x < $_cols} {incr x} {
+			set _chars($x,$y) $_chars($x,$next_y)
+		    }
+		    incr _cur_row -1
+		}
 	    }
 	}
     }