Make asciidoctor gem optional in tests

[Bo98]

Description

Everything not in the development group in the Gemfile appears to be optional. There is code elsewhere to skip these gems when they are not installed:

yard/spec/templates/markup_processor_integrations/integration_spec_helper.rb

Lines 27 to 31 in eddd10c

	before(:each) do
	if html_renderer.markup_class(markup).nil?
	skip "Missing markup renderer #{markup}"
	end
	end

yard/spec/i18n/locale_spec.rb

Lines 24 to 43 in eddd10c

	have_gettext_gem = true
	if RUBY_VERSION < "1.9"
	begin
	require "gettext/tools/poparser"
	rescue LoadError
	have_gettext_gem = false
	end
	else
	begin
	require "gettext/po_parser"
	rescue LoadError
	begin
	require "gettext/tools/poparser"
	rescue LoadError
	have_gettext_gem = false
	end
	end
	end
	it "returns true for existent PO", :if => have_gettext_gem do

This wasn't however the case for asciidoctor, which is currently required to be installed for the tests to pass.

This fixes the affected tests on Ruby 4.0 as asciidoctor doesn't support Ruby 4.0 upstream yet.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).