These commands were trusting that elaboration resulted in type-correct
terms, but users testing custom elaborators have found it to be
surprising that they do not do typechecking. This adds a `Meta.check`
step.
Closes#2736
See comment at `ExprDefEq.lean` for explanation.
Side effects:
- Improved error messages in two tests.
- Had to improve `getSuccesses` procedure at `App.lean`. It now
discards candidates that contain postponed elaboration problems.
If it is too disruptive for Mathlib, we should try to discard the
ones that have postponed metavariables.