This PR adds regression tests for `do`-notation issues that the new
elaborator fixes:
* `tests/elab/doNotation7.lean` collects reproducers for #2663, #2676,
#3126, #5607, #6426, and #8119.
* `tests/elab/12229.lean` covers the `logInfo` and `Std.TreeMap`
reproducers from #12229.
