Run the stdlib tests in presubmits

[Verdagon]

[Verdagon]

And make sure they dont supply a stdlib= mapping, so we can indirectly test that the stdlib is automatically brought in.

[tomerghelber]

Hey, this is a pretty important test. If we want to split development on stdlib I think we should push this forward.

[Verdagon]

I agree! It's on my shortlist https://github.com/ValeLang/Vale/issues?q=assignee%3AVerdagon+is%3Aopen so it'll be done soon, likely within 2 weeks, so you caught this at a good time.