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

src/nvim/testdir/runtest.vim

@@ -74,6 +74,9 @@ set nomore
 " Output all messages in English.
 lang mess C
 
+" Nvim: append runtime from build dir, which contains the generated doc/tags.
+let &runtimepath .= ','.expand($BUILD_DIR).'/runtime/'
+
 " Always use forward slashes.
 set shellslash
 

src/nvim/testdir/test_autocmd.vim

@@ -300,7 +300,6 @@ func Test_augroup_warning()
 endfunc
 
 func Test_BufReadCmdHelp()
-  helptags ALL
   " This used to cause access to free memory
   au BufReadCmd * e +h
   help
@@ -382,7 +381,6 @@ func Test_three_windows()
 
   only
 
-  helptags ALL
   help
   wincmd w
   1quit

src/nvim/testdir/test_cmdline.vim

@@ -242,7 +242,6 @@ func Test_getcompletion()
   let l = getcompletion('break', 'compiler')
   call assert_equal([], l)
 
-  helptags ALL
   let l = getcompletion('last', 'help')
   call assert_true(index(l, ':tablast') >= 0)
   let l = getcompletion('giveup', 'help')

src/nvim/testdir/test_taglist.vim

@@ -89,8 +89,11 @@ func Test_tagfiles()
 
   help
   let tf = tagfiles()
+  " Nvim: expectation(s) based on tags in build dir (added to &rtp).
+  "       Filter out the (non-existing) '../../../runtime/doc/tags'.
+  call filter(tf, 'filereadable(v:val)')
   call assert_equal(1, len(tf))
-  call assert_equal(fnamemodify(expand('$VIMRUNTIME/doc/tags'), ':p:gs?\\?/?'),
+  call assert_equal(fnamemodify(expand('$BUILD_DIR/runtime/doc/tags'), ':p:gs?\\?/?'),
 	\           fnamemodify(tf[0], ':p:gs?\\?/?'))
   helpclose
   call assert_equal(['Xtags1', 'Xtags2'], tagfiles())