Many of our tests in `tests/lean/run/` produce output from `#eval` (or
`#check`) statements, that is then ignored.
This PR tries to capture all the useful output using `#guard_msgs`. I've
only done a cursory check that the output is still sane --- there is a
chance that some "unchecked" tests have already accumulated regressions
and this just cements them!
In the other direction, I did identify two rotten tests:
* a minor one in `setStructInstNotation.lean`, where a comment says `Set
Nat`, but `#check` actually prints `?_`. Weird?
* `CompilerProbe.lean` is generating empty output, apparently indicating
that something is broken, but I don't know the signficance of this file.
In any case, I'll ask about these elsewhere.
(This started by noticing that a recent `grind` test file had an
untested `trace_state`, and then got carried away.)
Motivation: make sure the behavior is consistent with other arithmetic
operators.
This commit also removes the instance
```
instance : HMod (Fin n) Nat (Fin n) where
hMod := Fin.modn
```
because we have a coercion from `Fin n` to `Nat`.
Thus, given `a : Fin n` and `b : Nat`, `a % b` is ambiguous.
