Make `haddock_testsuite` respect `--test-accept`

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 (closed)