Wrong `could not execute: tasty-discover` (revisit after haskell/hie-bios#187)

When using a preprocessor (`tasty-discover`) for our tests, we get a wrong error from HLS, while compilation is fine.

See minimal repo to reproduce the issue: https://github.com/Sir4ur0n/hls-bug-preprocessor

* `stack test` works fine
* `haskell-language-server` returns an error:
```
File: /home/sir4ur0n/sandbox/hls-bug-preprocessor/test/Spec.hs
Hidden: no
Range: 1:0-2:0
Source: compiler
Severity: DsError
Message: haskell-language-server: could not execute: tasty-discover
Files that failed:
[INFO] finish: User TypeCheck (took 0.57s) * /home/sir4ur0n/sandbox/hls-bug-preprocessor/test/Spec.hs
```

While we **might** tell `hie.yaml` to ignore (cradle `none`) the `Spec.hs` file in particular, I don't see a reason why this even fails. HLS should support such files.

Thank you!