gdb/testsuite: rename _cur_x/_cur_y to _cur_col/_cur_row in lib/tuiterm.exp

gdb/testsuite: rename _cur_x/_cur_y to _cur_col/_cur_row in lib/tuiterm.exp

I am having trouble remembering which of _cur_x/_cur_y is columns and
which is rows, so renaming them helps.  We already have _rows and _cols
to represent the terminal size, so I think that makes sense to name the
"_cur" variables accordingly.

gdb/testsuite/ChangeLog:

	* lib/tuiterm.exp: Rename _cur_x/_cur_y to _cur_col/_cur_row.

Change-Id: I6abd3cdfdb295d8abde12dcd5f0ae09f18f07967