Make a test depending on the rubyzip gem optional. (#3177)

**What problem is this PR intended to solve?**

We manage the nokogiri's downstream RPM package on a Linux distro such
as CentOS 10 Stream and RHEL 10.
However, we don't want to manage the depending rubyzip gem RPM package
in CentOS 10 Stream and RHEL 10. And we still want to pass the
nokogiri's unit tests on the environments.

This PR is still to pass the nokogiri's unit tests where rubyzip is not
maintained.

**Have you included adequate test coverage?**

This PR is just to make a test case optional. So, it doesn't affect the
test coverage.

**Does this change affect the behavior of either the C or the Java
implementations?**

No. This PR is just for a test, not for an implementation.

test/xml/test_document_encoding.rb

@@ -91,7 +91,11 @@ class TestDocumentEncoding < Nokogiri::TestCase
         describe "pseudo-IO" do
           it "serializes correctly with Zip::OutputStream objects" do
             # https://github.com/sparklemotion/nokogiri/issues/2773
-            require "zip"
+            begin
+              require "zip"
+            rescue LoadError
+              skip("rubyzip is not installed")
+            end
 
             xml = <<~XML
               <?xml version="1.0" encoding="UTF-8"?>