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
We already did this, but missed a few cases due to the way arguments are
elaborated. So now, when checking an LHS, we don't allow LHS argument
types to be inferred from the pattern, but rather they must be inferred
from elsewhere. To do this, we keep track of the constraints which would
be solved when inferring the type, and make sure they don't solve any
new metavariables. Fixes#1510, and also now gets the error location
right as a bonus!
