test: Add screen test facility

- Add screen.lua which implements a remote screen to verify screen state by
  tests under functional/ui
- Add some basic screen/highlight tests

test/functional/ui/input_spec.lua

@@ -0,0 +1,40 @@
+local helpers = require('test.functional.helpers')
+local clear, execute, nvim = helpers.clear, helpers.execute, helpers.nvim
+local feed, next_message, eq = helpers.feed, helpers.next_message, helpers.eq
+
+describe('mappings', function()
+  local cid
+
+  local add_mapping = function(mapping, send)
+    local str = 'mapped '..mapping
+    local cmd = "nnoremap "..mapping.." :call rpcnotify("..cid..", 'mapped', '"
+                ..send:gsub('<', '<lt>').."')<cr>"
+    execute(cmd)
+  end
+
+  local check_mapping = function(mapping, expected)
+    feed(mapping)
+    eq({'notification', 'mapped', {expected}}, next_message())
+  end
+
+  before_each(function()
+    clear()
+    cid = nvim('get_api_info')[1]
+    add_mapping('<s-up>', '<s-up>')
+    add_mapping('<s-up>', '<s-up>')
+    add_mapping('<c-s-up>', '<c-s-up>')
+    add_mapping('<c-s-a-up>', '<c-s-a-up>')
+  end)
+
+  it('ok', function()
+    check_mapping('<s-up>', '<s-up>')
+    check_mapping('<c-s-up>', '<c-s-up>')
+    check_mapping('<s-c-up>', '<c-s-up>')
+    check_mapping('<c-s-a-up>', '<c-s-a-up>')
+    check_mapping('<s-c-a-up>', '<c-s-a-up>')
+    check_mapping('<c-a-s-up>', '<c-s-a-up>')
+    check_mapping('<s-a-c-up>', '<c-s-a-up>')
+    check_mapping('<a-c-s-up>', '<c-s-a-up>')
+    check_mapping('<a-s-c-up>', '<c-s-a-up>')
+  end)
+end)