build-system/julia: Enable tests.

* guix/build-system/julia.scm (julia-build): Set tests? default to #t.
* guix/build/julia-build-system.scm (check): Respect tests? and fix julia
  invocation.
  (%standard-phases): Add check phase after install.
* doc/guix.texi (julia-build-system): Update accordingly.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>