[gdb/testsuite] Fix linefeed scrolling in tuiterm

[gdb/testsuite] Fix linefeed scrolling in tuiterm

I came across a bug in the implementation of line feed in tuiterm, and added a
unit test that exposes it.

Before sending the line feed we have:
...
Screen Dump (size 8 columns x 4 rows, cursor at column 0, row 3):
    0 abcdefgh
    1 ijklmnop
    2 qrstuvwx
    3 yz01234
...
and after it we have:
...
Screen Dump (size 8 columns x 4 rows, cursor at column 0, row 1):
    0 ijklmnop
    1 qrstuvwx
    2 yz01234
    3 yz01234
...

Note how the cursor started at row 3 and after the line feed ended up at
row 1, while it should have stayed in row 3.

Fix this by moving "incr _cur_row -1" one level up in the loop nest in
proc _ctl_0x0a.

Tested on x86_64-linux.