[PATCH] Skip test_write_binary(GemSingletonTest) at rbs tests

	```
	Errno::EACCES: Permission denied @ rb_file_s_rename
	...
	D:/a/ruby/ruby/src/lib/rubygems/util/atomic_file_writer.rb:42:in 'File.rename'
	```

	It may caused with atomic_file_writer.rb

on `has_commit` check for the `backport` command.

I don't maintain local "master" branch on my ruby repository for stable
branch maintenance. I want just running `git fetch origin` to make it
work. It should work for those who pull origin/master into their local
master too.

Process::Status#exitstatus turn into nil when child process is signeled.
When exit_code was unchanged, test-bundled-gems.rb returned 0 and make was unable to detect the failure. Fix this.