Daniel, I'm also a bit ignorant as to why there are two different repos. Indeed putting the testsuite into the same repo as GHC seems to be the best option, but I'm assuming that since the GHC team has placed these projects into separate repos there must be a Good Reason to do so. Or perhaps there is no such reason? I really don't know - I'm just observing what is going on on the list and I see that something definitely is wrong with the workflow.
If you checkout a former ghc commit, how do you get a fitting test suite? Um... you don'?
If someone develops a feature inside of a branch and also changes/extends the test suite, does he than have a branch in the ghc repo and one in the test suite repo? That's what I'm doing. Not sure if there is an alternative.
Janek