Remove the declaration of Test::Unit::TestCase from test/helper.rb

Test::Unit::TestCase is defined in test/unit lib.
Remove test-unit-full from development requirements
Add tests for Emacs Lisp

"test ruby-backward-sexp" in test/ruby-mode-test.elk are
ported as enh-ruby-backward-sexp-test.

I didn't port "test ruby-begining-of-expr",
because it is commented-out and enh-ruby-beginning-of-expr is not
defined now.
Remove test/ruby-mode-test.elk
Introduce with-temp-enh-rb-buffer macro
Add Rakefile for running tests