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)
diff --git a/src/version.c b/src/version.c
index b43e393..4725e61 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1596,
+/**/
     1595,
 /**/
     1594,