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

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.