tests: use runtime from build for doc/tags with :help (#10479)

This is better practice in general, and allows to remove the "helptags
ALL" hacks.

Ref: https://github.com/neovim/neovim/issues/8824
Ref: https://github.com/neovim/neovim/commit/f1b67c3453c

* Makefile: fix dependencies with regard to helptags

- use the file as the main target to avoid unnecessary triggering
- use "make oldtest" on Travis to ensure it gets built

test/functional/autocmd/bufenter_spec.lua

@@ -33,9 +33,9 @@ describe('autocmd BufEnter', function()
   end)
 
   it('triggered by ":split normal|:help|:bw"', function()
+    helpers.add_builddir_to_rtp()
     command("split normal")
     command("wincmd j")
-    command("helptags runtime/doc")
     command("help")
     command("wincmd L")
     command("autocmd BufEnter normal let g:bufentered = 1")