While the discussion about how to refactor test framework is not
finished (#1654), make this change: move `rm -rf build` in the
beginning of the test. For these reasons:
* it is useful to inspect the contents of the `build` directory
especially after the test failure
* if build crashes mid-test (e.g. process killed), next run should
not be affected by the `build` directory from the previous run
This gives us the ability to define and use implementations locally, in
where clauses/local let bindings, as well as flag local definitions as
hints.
It's not yet quite equivalent to global hints, however, since it translated
the hint to a local let binding, which doesn't reduce, so if something
relies on the reduction behaviour of the hint, it won't work. This
refinement is coming later
