vc-list-files: make the stand-alone gnulib test work

vc-list-files: make the stand-alone gnulib test work

* modules/vc-list-files-tests (configure.ac):
Define and AC_SUBST abs_aux_dir.
(Makefile.am) [TESTS_ENVIRONMENT]: Rather than passing
$(abs_top_srcdir) to each script and having each of them
duplicate the work of setting PATH, set PATH here, using
the new variable, abs_aux_dir instead.
* tests/test-vc-list-files-cvs.sh: Don't set PATH here.
* tests/test-vc-list-files-git.sh: Likewise.
Reported by Bruno Haible.