tui: expose terminal type in 'term' option

Since "builtin" terminfo definitions were implemented (7cbf52db1b),
the decisions made by tui.c and terminfo.c are more relevant. Exposing
that decision in the 'term' option helps with troubleshooting.

Also: remove code that allowed setting t_Co. `:set t_Co=…` has never
worked; the highlight_spec test asserting that nvim_set_option('t_Co')
_does_ work makes no sense, and should not have worked.

test/functional/helpers.lua

@@ -603,6 +603,15 @@ local function get_pathsep()
   return funcs.fnamemodify('.', ':p'):sub(-1)
 end
 
+-- Returns a valid, platform-independent $NVIM_LISTEN_ADDRESS.
+-- Useful for communicating with child instances.
+local function new_pipename()
+  -- HACK: Start a server temporarily, get the name, then stop it.
+  local pipename = nvim_eval('serverstart()')
+  funcs.serverstop(pipename)
+  return pipename
+end
+
 local function missing_provider(provider)
   if provider == 'ruby' then
     local prog = funcs['provider#' .. provider .. '#Detect']()
@@ -732,6 +741,7 @@ local module = {
   missing_provider = missing_provider,
   alter_slashes = alter_slashes,
   hexdump = hexdump,
+  new_pipename = new_pipename,
 }
 
 return function(after_each)