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
Ideally we'd have a complete incremental build in CI, but that could be
a bit fiddly to set up at the moment (updating bootstrap code might make
it easier). This tests that the basic facilities work, though - there's
a lot can go wrong even in a small test like this, trust me, I have made
those mistakes :).
