patch 8.2.1856: "2resize" uses size of current window

Problem:    "2resize" uses size of current window.
Solution:   Use size of resized window. (Yasuhiro Matsumoto, closes #7152)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index a4b4f53..51d0722 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -1144,6 +1144,10 @@
   exe other_winnr .. 'resize 10'
   call assert_equal(10, winheight(other_winnr))
   call assert_equal(&lines - 10 - 3, winheight(0))
+  exe other_winnr .. 'resize +1'
+  exe other_winnr .. 'resize +1'
+  call assert_equal(12, winheight(other_winnr))
+  call assert_equal(&lines - 10 - 3 -2, winheight(0))
 
   %bwipe!
 endfunc