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/python3_spec.lua

@@ -97,6 +97,7 @@ describe('python3 provider', function()
   end)
 
   it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    helpers.add_builddir_to_rtp()
     source([=[
       python3 << EOF
       import vim