Adapt testsuite's reqlib modifier for cabal v2-build world
Currently the testsuite uses the user package database to find libraries for tests with reqlib
modifiers. However, with the advent of cabal v2-build
installation of things into the user package database is both difficult and discouraged.
To replace this we should either:
- take advantage of GHC's package environment file support
- or, use
cabal --store-dir=... v2-install --lib
to provide these packages. This will require updating the testsuite driver's ghc-pkg
logic.