Linter to check code style

[andrykonchin]

I am wondering whether it makes sense to automate some code style rules (specific to RubySpec codebase) with linter (e.g. Rubocop)? Even if it requires adding custom rules/cops...

For instance linter could detect:

• declaring new classes/global methods
• using -> {}.should_not raise_error
• test name duplication (issue Check in mspec that no 2 specs have the same name #664)

[eregon]

Currently RuboCop for ruby/spec is only run in this repo, not in CRuby, JRuby and TruffleRuby's copy of the specs.
It's kind of intentional, I don't want to bother people with overly strict style there, and it's usually trivial fixes when synchronizing.

I think for these 3 cases it's better in MSpec, in order:

• the global constants & classes check is already implemented (with CHECK_LEAKS=true). Global methods not yet though.
• could be a deprecation in MSpec (but need to fix usages in ruby/spec first to avoid many warnings)
• can be easily checked in MSpec (but needs to fix cases in ruby/spec first too)

[eregon]

Also, as much as possible, I'd also like to keep ruby/spec free of dependencies besides MSpec, including using such checks.