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.