Test --working-dir in the testsuite

When testing `./Setup` only, when `withDirectory` is used, instead of
changing into that directory when invoking processes, we now use the
`--working-dir` flag and keep a fixed CWD.

This will therefore passively test that `--working-dir` is working

In addition, it makes it possible to test things easily such as
`--working-dir` with a relative path as an argument. `cabal-install`
will only invoke `--working-dir` with an absolute path and hence is
isolated from any double interpretation issues.

Testing against these double interpretation issues is very important as
it also prevents over-interpretation of relative paths into absolute
paths. Passing absolute paths to tools such as hsc2hs can lead to the
build directory leaking into an interface file which leads to
non-reproducible results.