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 saves a lot of unnecessary exploring of size change graphs, which
can get over the top quite quickly if there's complex mutual
definitions, or even just a single function with an interesting variety
of recursive calls.
Fixes#1365Fixes#1277Fixes#645
