How to run the testsuite with an out of tree compiler?

I am trying to run `ghcide` on GHC's tests. The documentation says that I should pass `--test-compiler=..` to hadrian in order to do this. When I do this however it starts trying to configure a bunch of packages and starting to build stuff. I don't want hadrian to build anything, I just want it to use my patched version of GHC when running certain tests! How can I achieve this? Cheers, Matt