patch 8.2.1596: using win_screenpos('.') in tests works but is wrong

Problem:    Using win_screenpos('.') in tests works but is wrong.
Solution:   Use win_screenpos(0).
diff --git a/src/testdir/test_terminal3.vim b/src/testdir/test_terminal3.vim
index e982190..a3b8c1c 100644
--- a/src/testdir/test_terminal3.vim
+++ b/src/testdir/test_terminal3.vim
@@ -551,7 +551,7 @@
   let winpos = 50->getwinpos()
   call assert_equal(xroot, winpos[0])
   call assert_equal(yroot, winpos[1])
-  let [winrow, wincol] = win_screenpos('.')
+  let [winrow, wincol] = win_screenpos(0)
   let xoff = wincol * (has('gui_running') ? 14 : 7) + 100
   let yoff = winrow * (has('gui_running') ? 20 : 10) + 200
   call assert_inrange(xroot + 2, xroot + xoff, xpos)