patch 8.2.3989: some insert completion code is not tested

Problem:    Some insert completion code is not tested.
Solution:   Add a few tests.  Refactor thesaurus completion. (Yegappan
            Lakshmanan, closes #9460)

src/testdir/test_edit.vim

@@ -733,8 +733,13 @@ func Test_edit_CTRL_N()
     call feedkeys("Ii\<c-n>\<cr>\<esc>", "tnix")
     call feedkeys("ILO\<c-n>\<cr>\<esc>", 'tnix')
     call assert_equal(['INFER', 'loWER', 'infer', 'LOWER', '', ''], getline(1, '$'), e)
-
-    set noignorecase noinfercase complete&
+    set noignorecase noinfercase
+    %d
+    call setline(1, ['one word', 'two word'])
+    exe "normal! Goo\<C-P>\<C-X>\<C-P>"
+    call assert_equal('one word', getline(3))
+    %d
+    set complete&
     bw!
   endfor
 endfunc
@@ -900,6 +905,23 @@ func Test_edit_CTRL_T()
   bw!
 endfunc
 
+" Test thesaurus completion with different encodings
+func Test_thesaurus_complete_with_encoding()
+  call writefile(['angry furious mad enraged'], 'Xthesaurus')
+  set thesaurus=Xthesaurus
+  for e in ['latin1', 'utf-8']
+    exe 'set encoding=' .. e
+    new
+    call setline(1, 'mad')
+    call cursor(1, 1)
+    call feedkeys("A\<c-x>\<c-t>\<cr>\<esc>", 'tnix')
+    call assert_equal(['mad', ''], getline(1, '$'))
+    bw!
+  endfor
+  set thesaurus=
+  call delete('Xthesaurus')
+endfunc
+
 " Test 'thesaurusfunc'
 func MyThesaurus(findstart, base)
   let mythesaurus = [