test(cram): dune runtest on a disabled test

[Alizter]

Previously when a user tried to run a disabled cram test, set using the (enabled_if) field of the (cram) stanza, they would see dune succeed silently having skipped the test.

When this is part of dune runtest foo/ in a directory, this behaviour is fine. But if the user explicitly requested a disabled test then this behaviour can be confusing.

This PR introduces an error message when a user requests a disabled cram test, either via dune runtest or the test alias directly.

• changelog
• docs

[rgrinberg]

I don't think we want this behavior. Running a disabled explicitly should succeed.

[Alizter]

Hmm actually nevermind. I got confused about the enabled_if semantics here. I'll rethink this.

[rgrinberg]

I think that we all we need is for enabled_if to control whether the test is added to the runtest (or w/e it is set to) alias. That will give us the right behavior everywhere else as well.