add spec for TRUE/FALSE/NIL constants that are no longer defined

[lxxxvi]

Hello 👋

This PR covers

TRUE/FALSE/NIL constants are no longer defined.

from #823

I am uncertain if I found the right file to put the tests, can you please verify.
I hope I got the version guards right this time.

Again, thanks a lot.

[eregon]

It should be in language/predefined_spec.rb, could you move them there?
(There were specs for it incorrectly removed in b3ac8c8)

[eregon]

"is no longer defined" seems clearer

[eregon]

Suggested change

	FALSE FalseClass Synonym for false (deprecated in Ruby 3).
	FALSE FalseClass Synonym for false (deprecated, removed in Ruby 3).

(same for the others)

[eregon]

Even in 2.5 it was already deprecated, so it should be ruby_version_is ""..."3.0" do

[lxxxvi]

Good point, thank you!

[lxxxvi]

@eregon Thanks for the hint. I moved and adopted the specs accordingly.

Right now it is structured like this:

describe "The predefined global constants" do
  describe "TRUE" do
    ruby_version_is "3.0" {}
    ruby_version_is ""..."3.0" {}
  end

  describe "FALSE" do
    ruby_version_is "3.0" {}
    ruby_version_is ""..."3.0" {}
  end

  describe "NIL" do
    ruby_version_is "3.0" {}
    ruby_version_is ""..."3.0" {}
  end

  # ...
end

Would it make sense to do it that way:

describe "The predefined global constants" do
  ruby_version_is "3.0" do
    describe "TRUE" {}
    describe "FALSE" {}
    describe "NIL" {}
  end

  ruby_version_is ""..."3.0" do
    describe "TRUE" {}
    describe "FALSE" {}
    describe "NIL" {}
  end

  # ...
end

[eregon]

Looks great, thanks!

[eregon]

Re structure I like it as-is for consistency.
Duplication in specs is not necessarily a bad thing, especially trivial things like guards.