patch 8.2.1589: term_start() options for size are overruled by 'termwinsize'

Problem:    Term_start() options for size are overruled by 'termwinsize'.
            (Sergey Vlasov)
Solution:   Set 'termwinsize' to the specified size.
diff --git a/src/testdir/term_util.vim b/src/testdir/term_util.vim
index 17f9752..4a5e649 100644
--- a/src/testdir/term_util.vim
+++ b/src/testdir/term_util.vim
@@ -73,7 +73,8 @@
   set t_Co=256 background=light
   hi Normal ctermfg=NONE ctermbg=NONE
 
-  " Make the window 20 lines high and 75 columns, unless told otherwise.
+  " Make the window 20 lines high and 75 columns, unless told otherwise or
+  " 'termwinsize' is set.
   let rows = get(a:options, 'rows', 20)
   let cols = get(a:options, 'cols', 75)
   let statusoff = get(a:options, 'statusoff', 1)
@@ -86,11 +87,12 @@
 
   let cmd = GetVimCommandCleanTerm() .. reset_u7 .. a:arguments
 
-  let options = {
-	\ 'curwin': 1,
-	\ 'term_rows': rows,
-	\ 'term_cols': cols,
-	\ }
+  let options = #{curwin: 1}
+  if &termwinsize == ''
+    let options.term_rows = rows
+    let options.term_cols = cols
+  endif
+
   " Accept other options whose name starts with 'term_'.
   call extend(options, filter(copy(a:options), 'v:key =~# "^term_"'))