Skip `test_gem_package_task` if `rake` not present

Instead of erroring.

test/rubygems/test_gem_package_task.rb

@@ -1,7 +1,16 @@
 # frozen_string_literal: true
 require 'rubygems/test_case'
 require 'rubygems'
-require 'rubygems/package_task'
+
+begin
+  require 'rubygems/package_task'
+rescue LoadError => e
+  raise unless e.path == 'rake/packagetask'
+end
+
+unless defined?(Rake::PackageTask)
+  warn 'Skipping Gem::PackageTask tests.  rake not found.'
+end
 
 class TestGemPackageTask < Gem::TestCase
 
@@ -107,4 +116,4 @@ class TestGemPackageTask < Gem::TestCase
     assert_equal 'pkg/nokogiri-1.5.0-java', pkg.package_dir_path
   end
 
-end
+end if defined?(Rake::PackageTask)