Allow building hadrian without QuickCheck-dependent selftest rules

Motivation

The hadrian executable depends on QuickCheck for building, meaning this library (and its dependencies) will need to be built for bootstrapping GHC in the future. Building QuickCheck and its dependencies does requires TemplateHaskell, however, which can be problematic: E.g. when building a statically linking GHC toolchain, TemplateHaskell can be tricky to get to work, and cross-compiling TemplateHaskell doesn't work at all without -fexternal-interpreter.

While these scenarios are maybe not super common to come up when bootstrapping GHC, it is an element of fragility we may want to avoid from a bootstrapping perspective (e.g. in nixpkgs master, the bootstrap GHCs are (atm) unable to compile code using TemplateHaskell). It also further solidifies the status quo that GHC can't be bootstrapped with anything but GHC.

As far as I can tell, hadrian only needs QuickCheck for its selftest rules which are probably not necessary for just building GHC, so they are only necessary when working on GHC / hadrian itself.

Proposal

Add a Cabal flag to hadrian.cabal that gates building Rules.Selftest and the QuickCheck dependency. It should default to false and only be active when building hadrian within the GHC repository.