The `req_interp` test attribute is broken for --test-compiler=stage1
## Summary
Running `hadrian/build --flavour=devel2+debug_ghc+debug_info -j6 test --test-compiler=stage1 --docs=none` I get a very large amount of test failures related to missing external interpreter/missing interpreter.
Would be convenient if that would respect the req_interp flag and not try to run all those tests.
## Environment
* GHC version used: any
Optional:
* Operating System: linux
* System Architecture: x86
issue