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/ex_cmds/help_spec.lua

@@ -9,6 +9,7 @@ describe(':help', function()
   before_each(clear)
 
   it('window closed makes cursor return to a valid win/buf #9773', function()
+    helpers.add_builddir_to_rtp()
     command('help help')
     eq(1001, funcs.win_getid())
     command('quit')