vim-patch:8.1.2066: no tests for state()

Problem:    No tests for state().
Solution:   Add tests.  Clean up some feature checks.  Make "a" flag work.

c258549032

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/functional/vimscript/state_spec.lua

@@ -0,0 +1,69 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local exec = helpers.exec
+local exec_lua = helpers.exec_lua
+local feed = helpers.feed
+local meths = helpers.meths
+local poke_eventloop = helpers.poke_eventloop
+
+before_each(clear)
+
+describe('state() function', function()
+  it('works', function()
+    meths.ui_attach(80, 24, {})  -- Allow hit-enter-prompt
+
+    exec_lua([[
+      function _G.Get_state_mode()
+        _G.res = { vim.fn.state(), vim.api.nvim_get_mode().mode:sub(1, 1) }
+      end
+
+      function _G.Run_timer()
+        local timer = vim.uv.new_timer()
+        timer:start(0, 0, function()
+          _G.Get_state_mode()
+          timer:close()
+        end)
+      end
+    ]])
+    exec([[
+      call setline(1, ['one', 'two', 'three'])
+      map ;; gg
+      func RunTimer()
+        call timer_start(0, {id -> v:lua.Get_state_mode()})
+      endfunc
+      au Filetype foobar call v:lua.Get_state_mode()
+    ]])
+
+    -- Using a ":" command Vim is busy, thus "S" is returned
+    feed([[:call v:lua.Get_state_mode()<CR>]])
+    eq({ 'S', 'n' }, exec_lua('return _G.res'))
+
+    -- Using a timer callback
+    feed([[:call RunTimer()<CR>]])
+    poke_eventloop()  -- Allow polling for events
+    eq({ 'c', 'n' }, exec_lua('return _G.res'))
+
+    -- Halfway a mapping
+    feed([[:call v:lua.Run_timer()<CR>;]])
+    meths.get_mode()  -- Allow polling for fast events
+    feed(';')
+    eq({ 'mS', 'n' }, exec_lua('return _G.res'))
+
+    -- Insert mode completion
+    feed([[:call RunTimer()<CR>Got<C-X><C-N>]])
+    poke_eventloop()  -- Allow polling for events
+    feed('<Esc>')
+    eq({ 'aSc', 'i' }, exec_lua('return _G.res'))
+
+    -- Autocommand executing
+    feed([[:set filetype=foobar<CR>]])
+    eq({ 'xS', 'n' }, exec_lua('return _G.res'))
+
+    -- messages scrolled
+    feed([[:call v:lua.Run_timer() | echo "one\ntwo\nthree"<CR>]])
+    meths.get_mode()  -- Allow polling for fast events
+    feed('<CR>')
+    eq({ 'Ss', 'r' }, exec_lua('return _G.res'))
+  end)
+end)