patch 8.2.1865: Vim9: add() does not check type of argument

Problem:    Vim9: add() does not check type of argument.
Solution:   Inline the add() call. (closes #7160)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index cc6af6a..bded91f 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1772,6 +1772,24 @@
   list2str(l, true)->assert_equal(s)
 enddef
 
+def Test_list_add()
+  var l: list<number>  # defaults to empty list
+  add(l, 9)
+  assert_equal([9], l)
+
+  var lines =<< trim END
+      var l: list<number>
+      add(l, "x")
+  END
+  CheckDefFailure(lines, 'E1012:', 2)
+
+  lines =<< trim END
+      var l: list<number> = test_null_list()
+      add(l, 123)
+  END
+  CheckDefExecFailure(lines, 'E1130:', 2)
+enddef
+
 def SID(): number
   return expand('<SID>')
           ->matchstr('<SNR>\zs\d\+\ze_$')