vim-patch:8.2.3321: some code is not tested

Problem:    Some code is not tested.
Solution:   Add some more tests. (Dominique Pellé, closes vim/vim#8735)
bd9e796125

Include Test_confirm_write_partial_file() anyway, even though it will
not be run.

src/nvim/testdir/test_writefile.vim

@@ -191,6 +191,14 @@ func Test_saveas()
   close!
   enew | only
   call delete('Xfile')
+
+  " :saveas should detect and set the file type.
+  syntax on
+  saveas! Xsaveas.pl
+  call assert_equal('perl', &filetype)
+  syntax off
+  %bw!
+  call delete('Xsaveas.pl')
 endfunc
 
 func Test_write_errors()