Simplify Rubocop test case

No need to test rubylint again here
Use let bindings instead of hooks to set variables
Make Emacs pick the right shell right away

Add a proper shebang to the shell script resources, so that Emacs
automatically picks the right shell, making our sh-set-shell calls
superfluous.
Remove superfluous test helper
Use Emacs Lisp in some selection tests

We don't need Python here, so lets use Emacs Lisp
Silence Python mode in the tests
[Fix #287] Add option to disable style checks in Rubocop
Fix grammar and wording