The test seems to fail sometimes due to a warning message of circular
require on very slow machine.

```
  1) Failure:
TestDigest::TestDigestParen#test_race_mixed [/export/home/rubyci/unstable10s/tmp/build/20200326T091911Z/ruby/test/digest/test_digest.rb:263]:
assert_separately failed with error message
pid 12458 exit 0
| /export/home/rubyci/unstable10s/tmp/build/20200326T091911Z/ruby/.ext/common/digest.rb:17: warning: /export/home/rubyci/unstable10s/tmp/build/20200326T091911Z/ruby/.ext/common/digest.rb:17: warning: loading in progress, circular require considered harmful - /export/home/rubyci/unstable10s/tmp/build/20200326T091911Z/ruby/test/digest/digest/foo.rb
| 	from -:10:in  `block (2 levels) in <main>'
| 	from /export/home/rubyci/unstable10s/tmp/build/20200326T091911Z/ruby/.ext/common/digest.rb:17:in  `const_missing'
| 	from /export/home/rubyci/unstable10s/tmp/build/20200326T091911Z/ruby/.ext/common/digest.rb:17:in  `require'
|
```
https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable10s/ruby-master/log/20200326T091911Z.fail.html.gz