[Bug #21718]

Otherwise objects that don't define it, but define a fairly liberal
`method_missing` method will run into errors that are hard to understand:

```ruby
class Foo
  def method_missing(name, ...)
    name
  end
end

p Foo.new.inspect
```

```
'Kernel#inspect': wrong argument type Symbol (expected Array) (TypeError)
	from ../test.rb:7:in '<main>'
```

Make Kernel#inspect ask which instance variables should be dumped by
the result of `#instance_variables_to_inspect`.

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62656 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* Other ruby implementations use the spec/ruby directory.
  [Misc #13792] [ruby-core:82287]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e