Follow-up to 8969efca8 (Vim patch 8.1.0723)

NOTE: This changes the main entrypoint for running single oldtest files
to not use/require the ".res" extension anymore.  But it is handled for
B/C.

Adds a phony rule to run oldtest by filename.
Not going through "$(MAKE)" avoids GNUmakefile being used then (which I
use for WIP things), and it seems like SINGLE_MAKE should be used anyway
probably.

Problem:    Cannot run specific test when in src/testdir the same was as in
            the src directory.
Solution:   Move build rule to src/testdir/Makefile.
https://github.com/vim/vim/commit/ec50401e1e1357a1340b3c92109fd4860e38a8ac

Developer can omit '.res' suffix now.
TEST_FILE=test_syntax make oldtest
or
make -C src/nvim/testdir test_syntax

Problem:    "make clean" in top dir does not cleanup indent test output.
Solution:   Clean the indent test output.  Do not rely on the vim executable
            for that. (closes vim/vim#4307)
https://github.com/vim/vim/commit/e13a3901cae0afb4d2af30d497696af08029fd81

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

A separate rule it clearer, and allows for `make --keep-going lint` in
general later.

Ref: https://github.com/neovim/neovim/pull/10714

Ref: https://github.com/neovim/neovim/pull/10714#issuecomment-518932355

This avoids errors when using "make lint", but "flake8" is not
available.  We do not want to install it then via third-party.
On CI "make pylint" is used explicitly.

While not doing any harm with "Unix Makefiles", they do not work there
as-is.  Therefore just do not use them then.

Followup to #10366 (7f6ff829a).