Alec Theriault authored May 29, 2019 and Marge Bot committed Jun 11, 2019

Suppose you've made changes that affect the output of `haddockHtmlTest`
so that the following is failing:

    ./hadrian/build.sh -c --only=haddockHtmlTest test

Then, the following will accept new output for Haddock's test cases.

    ./hadrian/build.sh -c --only=haddockHtmlTest test --test-accept

You still do need to make sure those new changes (which show up in
Haddock's tree) get committed though.

Fixes #16694