[WIP] Add specs for spec output

[straight-shoota]

This a work in progress trying to add specs for spec output.

Until know, there are no specs for this at all. Unfortunately the spec implementation is not very modular, so I had to mock it quite extensively to be able to run it in a spec context. But it work and required only a minimal change in the actual implementation.

It's far from ready but I'd like to get some feedback on this approach.

This could be useful for implementing #5633

[RX14]

Honestly i'd advocate for invoking bin/crystal and on some example spec files and observing the output over all this stubbing.

[luislavena]

Honestly i'd advocate for invoking bin/crystal and on some example spec files and observing the output over all this stubbing.

On separate issue, @asterite has commented that he would prefer programmatically initializing/using code than shelling out externally. That is also good considering shelling out might be expensive on other platforms (ie. Windows).

IMO the excessive stubbing is caused by the lack of internal API on Spec, in which case can be resolved in future PRs.

[straight-shoota]

It would certainly make sense to have some integration test running bin/crystal spec - to ensure it really works as expected.
But It think programmatic testing is preferable in general to validate specific behaviour. And I guess this could easily be improved by refactoring some tight couplings out of Spec.

[jhass]

Where do we want to go with this? I think given what this tries to cover, the amount of added complexity is actually pretty fair. So I would be willing to merge this in as a basis. Then if somebody has the idea and time to reduce the stubbing or find a better approach in general they can update this.