Fix YAML spec for libyaml 0.2.1

[asterite]

Fixes #6283

[straight-shoota]

I don't think this is a good fix because it can hide real errors.

[asterite]

@straight-shoota Suggestions for a good fix? :-)

There's nothing in YAML that can generate "...\n" at the end of the document other than a document. I really don't think we should put a lot more effort into this.

[straight-shoota]

Take a look at the affected examples. There are very similar examples, which sometimes expect ...\n, sometimes not.

I'd suggest to add an argument flag expect_document_end to assert_yaml_document/assert_built which appends ...\n to the expected string if yaml_get_version is <0.2.1.

[asterite]

Sounds good, but I don't have more time for Crystal today, so someone else should send a PR.

[asterite]

It's probably more correct to change the expected string based on the YAML version, so that's another solution. Of course, if later in libyaml 0.2.2 they revert that change because it was a mistake, our code will have to change again.

[straight-shoota]

Actually, it would probably even be better to normalize the results from YAML::Builder itself. We shouldn't expect different output depending on libyaml version. I expect we will have support these different libyaml versions for quite some time.