patch 8.2.4692: no test for what 8.2.4691 fixes

Problem:    No test for what 8.2.4691 fixes.
Solution:   Add a test.  Use a more generic sotlution. (closes #10090)

src/testdir/test_mapping.vim

@@ -1566,4 +1566,37 @@ func Test_plug_remap()
   %bw!
 endfunc
 
+" Test for mapping <LeftDrag> in Insert mode
+func Test_mouse_drag_insert_map()
+  set mouse=a
+  func ClickExpr()
+    call test_setmouse(1, 1)
+    return "\<LeftMouse>"
+  endfunc
+  func DragExpr()
+    call test_setmouse(1, 2)
+    return "\<LeftDrag>"
+  endfunc
+  inoremap <expr> <F2> ClickExpr()
+  imap <expr> <F3> DragExpr()
+
+  inoremap <LeftDrag> <LeftDrag><Cmd>let g:dragged = 1<CR>
+  exe "normal i\<F2>\<F3>"
+  call assert_equal(1, g:dragged)
+  call assert_equal('v', mode())
+  exe "normal! \<C-\>\<C-N>"
+  unlet g:dragged
+
+  inoremap <LeftDrag> <LeftDrag><C-\><C-N>
+  exe "normal i\<F2>\<F3>"
+  call assert_equal('n', mode())
+
+  iunmap <LeftDrag>
+  iunmap <F2>
+  iunmap <F3>
+  delfunc ClickExpr
+  delfunc DragExpr
+  set mouse&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab