Avoid ever creating .deps directory

Thanks to @jszakmeister this uses make own features.
test1.in is not a test and it must be run always

test1.in generates files like small.vim, tiny.vim, mbyte.vim, mzscheme.vim,
lua.vim which are then used by other tests. So it must be run always.
Use MAKEOVERRIDES to pass SCRIPTS/TESTNUM arguments for `make test`
Make indentation consistent: use 2-space indent always
Merge pull request #845 'Testdir and top-level Makefile improvements'