build: Support running the test suite from a directory with symlinks.

Fixes <http://bugs.gnu.org/17935>.
Reported by Alex Kost <alezost@gmail.com>.

* test-env.in: Canonicalize $NIX_STORE_DIR, and remove
  $NIX_IGNORE_SYMLINK_STORE setting.
* tests/guix-register.sh: Likewise, canonicalize $new_store_dir and
  $new_store and leave $NIX_IGNORE_SYMLINK_STORE unchanged.