build-system: emacs: Add improved check phase.
* guix/build-system/emacs.scm (emacs-build): Add #:test-command keyword
argument. Remove #:configure-flags and #:test-target keyword arguments.
* guix/build/emacs-build-system.scm (check): New procedure.
(%standard-phases): Register check phase after the build phase.
Signed-off-by: Arun Isaac <arunisaac@systemreboot.net>