This is in similar vein Rake is optionally loaded in:

https://github.com/ruby/rdoc/blob/5437418baf8eeb984fe7e74b181d3da1b174d0f7/test/rdoc/test_rdoc_task.rb#L3-L6

https://github.com/ruby/rdoc/commit/5f3901ae60

https://github.com/ruby/rdoc/commit/b1e7129e05

https://rubyci.org/logs/rubyci.s3.amazonaws.com/scw-ad7f67/ruby-master/log/20200425T175405Z.fail.html.gz
```
  4) Error:
TestRDocServlet#test_asset:
NoMethodError: undefined method `replace' for nil:NilClass
    /home/chkbuild/build/20200425T175405Z/ruby/test/rdoc/test_rdoc_servlet.rb:48:in `setup'

  5) Error:
TestRDocServlet#test_asset:
NoMethodError: undefined method `replace' for nil:NilClass
    /home/chkbuild/build/20200425T175405Z/ruby/test/rdoc/test_rdoc_servlet.rb:63:in `teardown'
```
...
```
 74) Error:
TestRDocServlet#test_store_for_site:
NoMethodError: undefined method `replace' for nil:NilClass
    /home/chkbuild/build/20200425T175405Z/ruby/test/rdoc/test_rdoc_servlet.rb:48:in `setup'

 75) Error:
TestRDocServlet#test_store_for_site:
NoMethodError: undefined method `replace' for nil:NilClass
    /home/chkbuild/build/20200425T175405Z/ruby/test/rdoc/test_rdoc_servlet.rb:63:in `teardown'
```

This reverts commit ca5812fe4516a10cc687281f9e47e1a08449f1ab.

Now tool/lib/minitest provides "omit", so it should work.

This reverts commit b4da6fc1c277190bbd10e795ebf3be45772038e8.

`make test-all` uses minitest, which led to "undefined method `omit'"

https://github.com/ruby/rdoc/commit/1c5bf2ae1d