Dir.mktmpdir and File.join accept Pathname

[znz]

File.join uses FilePathStringValue, so it accepts Pathname.

https://github.com/ruby/ruby/blob/7c2660b34774c017accff8a7d78371c2f368b9dc/file.c#L5160

Dir.mktmpdir uses File.join internally, so it also accepts Pathname as parent.
And it use String.try_convert for prefix_suffix, so they accept string instead of String.

https://github.com/ruby/ruby/blob/923f831804b8bc26bf0412b932791d9090ff8ac4/lib/tmpdir.rb#L159-L165

[soutaro]

@znz Thanks! Can you add some tests to File_test.rb?

We don't have good place for Dir.mktmpdir tests. It will be really great if you can rewrite the test cases in Dir_tmpdir_test.rb. (It has only two test methods, not too difficult.)