test: Nokogumbo's contract expectations

See rubys/nokogumbo#171 which introduced
forward-looking behavior to integrate with a future Nokogiri::HTML5
after the Nokogumbo merger.

These tests, which are failing right now, will drive much of the rest
of the integration work.

lib/nokogiri/version/info.rb

@@ -190,6 +190,10 @@ def self.uses_libxml?(requirement = nil) # :nodoc:
     Gem::Requirement.new(requirement).satisfied_by?(VersionInfo.instance.loaded_libxml_version)
   end
 
+  def self.uses_gumbo?
+    uses_libxml? # TODO: replace with Gumbo functionality
+  end
+
   def self.jruby? # :nodoc:
     VersionInfo.instance.jruby?
   end

test/test_nokogumbo_contract.rb

@@ -0,0 +1,26 @@
+require "helper"
+
+describe "Nokogumbo contract expectations" do
+  # per https://github.com/rubys/nokogumbo/pull/171
+  it "includes the HTML5 public interface" do
+    skip("Gumbo is not supported on this platform") unless Nokogiri.uses_gumbo?
+
+    assert_includes(::Nokogiri.singleton_methods, :HTML5)
+
+    assert_equal(defined?(::Nokogiri::HTML5), "constant")
+    assert_includes(::Nokogiri::HTML5.singleton_methods, :parse)
+    assert_includes(::Nokogiri::HTML5.singleton_methods, :fragment)
+
+    assert_equal(defined?(::Nokogiri::HTML5::Node), "constant")
+    assert_equal(defined?(::Nokogiri::HTML5::Document), "constant")
+    assert_equal(defined?(::Nokogiri::HTML5::DocumentFragment), "constant")
+  end
+
+  it "includes a replacement for the Nokogumbo private interface" do
+    skip("Gumbo is not supported on this platform") unless Nokogiri.uses_gumbo?
+
+    assert_equal(defined?(::Nokogiri::Gumbo), "constant")
+    assert_includes(::Nokogiri::Gumbo.singleton_methods, :parse)
+    assert_includes(::Nokogiri::Gumbo.singleton_methods, :fragment)
+  end
+end