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

ci/common/test.sh

@@ -113,7 +113,7 @@ run_functionaltests() {(
 run_oldtests() {(
   enter_suite oldtests
   ulimit -c unlimited || true
-  if ! make -C "${TRAVIS_BUILD_DIR}/src/nvim/testdir"; then
+  if ! make oldtest; then
     reset
     fail 'oldtests' F 'Legacy tests failed'
   fi