vim-patch:8.2.1022: various parts of code not covered by tests

Problem:    Various parts of code not covered by tests.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6300)

845e0ee594

Omit test_iminsert.vim: the commit that created this file was N/A.
Omit test_viminfo.vim: the added tests are N/A.

test/functional/legacy/digraph_spec.lua

@@ -0,0 +1,46 @@
+local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
+local clear = helpers.clear
+local feed = helpers.feed
+
+before_each(clear)
+
+describe('digraph', function()
+  -- oldtest: Test_entering_digraph()
+  it('characters displayed on the screen', function()
+    local screen = Screen.new(10, 6)
+    screen:set_default_attr_ids({
+      [0] = {bold = true, foreground = Screen.colors.Blue},  -- NonText
+      [1] = {foreground = Screen.colors.Blue},  -- SpecialKey
+      [2] = {bold = true},  -- ModeMsg
+    })
+    screen:attach()
+    feed('i<C-K>')
+    screen:expect([[
+      {1:^?}           |
+      {0:~           }|
+      {0:~           }|
+      {0:~           }|
+      {0:~           }|
+      {2:-- INSERT -} |
+    ]])
+    feed('1')
+    screen:expect([[
+      {1:^1}           |
+      {0:~           }|
+      {0:~           }|
+      {0:~           }|
+      {0:~           }|
+      {2:-- INSERT -} |
+    ]])
+    feed('2')
+    screen:expect([[
+      ½^           |
+      {0:~           }|
+      {0:~           }|
+      {0:~           }|
+      {0:~           }|
+      {2:-- INSERT -} |
+    ]])
+  end)
+end)