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

[Matthew Pickering]

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