test: port Test_edit_08() to Lua test

Test_edit_08() depends on special-case handling in has_compl_option()
and redrawing() which are in Vim but are not wanted in Nvim. Using a Lua
test instead of depending on workarounds in the core to make the VimL
test work.

src/nvim/testdir/test_edit.vim

@@ -217,6 +217,7 @@ func! Test_edit_07()
 endfunc
 
 func! Test_edit_08()
+  throw 'skipped: moved to test/functional/legacy/edit_spec.lua'
   " reset insertmode from i_ctrl-r_=
   new
   call setline(1, ['abc'])

test/functional/legacy/edit_spec.lua

@@ -0,0 +1,25 @@
+-- Test for edit functions
+-- See also: src/nvim/testdir/test_edit.vim
+
+local helpers = require('test.functional.helpers')(after_each)
+local source = helpers.source
+local eq, eval = helpers.eq, helpers.eval
+local funcs = helpers.funcs
+local clear = helpers.clear
+
+describe('edit', function()
+  before_each(clear)
+
+  it('reset insertmode from i_ctrl-r_=', function()
+    source([=[
+      call setline(1, ['abc'])
+      call cursor(1, 4)
+      call feedkeys(":set im\<cr>ZZZ\<c-r>=setbufvar(1,'&im', 0)\<cr>",'tnix')
+    ]=])
+    eq({'abZZZc'}, funcs.getline(1,'$'))
+    eq({0, 1, 1, 0}, funcs.getpos('.'))
+    eq(0, eval('&im'))
+  end)
+
+end)
+