patch 8.2.1766: Vim9: Some tests are still using :let

Problem:    Vim9: Some tests are still using :let.
Solution:   Change the last few declarations to use :var.
diff --git a/src/testdir/test_maparg.vim b/src/testdir/test_maparg.vim
index b2ace6f..ea921d6 100644
--- a/src/testdir/test_maparg.vim
+++ b/src/testdir/test_maparg.vim
@@ -83,13 +83,13 @@
 
 def Test_vim9_maparg()
   nmap { w
-  let one: string = maparg('{')
+  var one: string = maparg('{')
   assert_equal('w', one)
-  let two: string = maparg('{', 'n')
+  var two: string = maparg('{', 'n')
   assert_equal('w', two)
-  let three: string = maparg('{', 'n', 0)
+  var three: string = maparg('{', 'n', 0)
   assert_equal('w', three)
-  let four: dict<any> = maparg('{', 'n', 0, 1)
+  var four: dict<any> = maparg('{', 'n', 0, 1)
   assert_equal(['{', 'w', 'n'], [four.lhs, four.rhs, four.mode])
   nunmap {
 enddef