Extract Base64.encode64 method into test case

[hsbt]

What was the end-user or developer problem that led to this PR?

I have a plan to promote base64 as bundled gems at ruby/ruby. see https://bugs.ruby-lang.org/issues/19351

After that, the test suite of rubygems is broken because it couldn't load base64. So, we need to avoid to use Base64 library at our tests.

What is your fix for the problem, implemented in this PR?

Base64.encode64 is simple pure ruby method. I ported it to test_gem_request.rb.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[simi]

Would be an alternative to add base64 gem into test Gemfile?

[hsbt]

Would be an alternative to add base64 gem into test Gemfile?

Yes, but It's only for this repository. rubygems tests of ruby/ruby repository is failed after extracting base64 from its repository.