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/provider/ruby_spec.lua

@@ -101,6 +101,7 @@ end)
 
 describe('ruby provider', function()
   it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    helpers.add_builddir_to_rtp()
     command([=[autocmd BufDelete * ruby VIM::evaluate('expand("<afile>")')]=])
     feed_command('help help')
     eq(2, eval('1+1'))  -- Still alive?