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WEBrick tests are failing at the moment.
From the stack trace, you see that webrick/config.rb:14 is requiring webrick/utils, but we are picking up the one in the test tree (https://github.com/jruby/jruby/blob/e86a6b53c016e6c267865fdb49b93bdc729030d3/test/mri/webrick/utils.rb) instead of the one in the lib tree.
I suspect that f98f362 added tests that exposes this issue.
I poked around MRI's test run, and it appears they explicitly pass -I pointing at lib and . before going on to run the tests. This is probably the difference; we just rely on them being in load path by default, but load path is searched after -I paths on the command line.
Testing this theory now.
That appears to be working locally. I'll make sure I have a good local test:mri run and then push.
Add appropriate -I flags to test:mri to fix #1380.