Update CI testscript

  - Avoid unnecessary rebuild of dependencies caused by the artificial
    separation between "prepare" and "build" (wasn't able to figure out
    what causes Stack to do that)

  - Avoid spurious warnings caused by missing configure and
    HsDirectoryConfig.h.in by running autoreconf earlier

  - Use extra-deps instead of ad hoc approach to override dependencies

  - The previous change requires creating the stack.yaml manually, which
    ends up being simpler than using "stack init" (only downside is that
    you have to specify the full LTS version, not just the major
    version).

  - Remove caching in AppVeyor for now.  Having four jobs means that
    there's not enough room in the cache, apparently causing strange
    build failures due to corrupt cache files being loaded.

  - Refactor the testscript slightly