add spec for Range objects to be frozen

[lxxxvi]

Hello 👋

From #823 :

Regexp literals and all Range objects are frozen. [Feature #8948] [Feature #16377] [Feature #15504]

/foo/.frozen? #=> true
(42...).frozen? # => true

I think spec for frozen Regexp literals already exists:

spec/language/regexp_spec.rb

Lines 21 to 25 in 136a6ab

	ruby_version_is "3.0" do
	it "is frozen" do
	/Hello/.should.frozen?
	end
	end

So this PR only adds the missing spec for Range objects.

[eregon]

Thanks!