spock-example | Spock example specifications along with ready-to-go Gradle | Plugin library

Okay, I have taken a look at your project. As you said, it is just the Spock sample project, upgraded to run Spock 2 tests. BTW, it still should be upgraded further, because in the current configuration compilation does not work with current Java versions, but that is off topic here, I am just mentioning it because I ran into a problem and then downgraded to Java 8 in order quickly reproduce your actual problem. The JUnit test's package name is not the problem either, even though the default package is always ugly, just like for the Spock tests.

Spock 1.x is based on JUnit 4, but Spock 2.x is based on JUnit 5 platform. This is also the one automatically found by Surefire when analysing the project dependencies. If you want Surefire to run multiple engines in parallel, you need to configure the corresponding providers as plugin dependencies, as mentioned here.

In your case, just add this to the POM:

Due to incompatible versions of dependencies, Spock won't build with GHC 8.8 and above. A similar problem is described in Spock issue #149, though I'm not fully sure it is exactly the same incompatibility. The error you got from Stack hints at that, as base-4.13.0.0 is the version of base that is bundled with GHC 8.8. cabal-install failed in a more obscure way because, upon noting the incompatibility, it tries to solve the dependencies using older versions of Spock, eventually picking 0.9.0.1, attempting and, thanks to a missing version upper bound for the reroute dependency, failing to build it.

(Shortly after this answer was posted, the missing upper bound was retrofitted to the old Spock version, so attempting to reproduce the problem now will lead to an easier to understand failure.)

Casting the tutorial aside, the most straightforward way to use Spock given those complications is probably through cabal-install 3+. Begin by using ghcup to switch to GHC 8.6.5: