Clean up generated and copied files

[nobu]

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

leaving the files after gem rebuild was intentional, for local
inspection, but the test suite should be made to clean up after
itself independently of that.

#4913 (comment)

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

Clean up generated and copied files.

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

[deivid-rodriguez]

@duckinator Can you have a second look here?

[hsbt]

The current test code still remains some of garbages at ruby/ruby repo. I will merge this.

@duckinator If you have some concerns about this, can you submit another PR? Thanks.

[duckinator]

This looks good to me. Thanks for taking care of this, @nobu + @hsbt -- I've been really busy this month, and wound up missing this.