OAS3 integration tests and fixes SO-103

[XVincentX]

• Adds some integration tests on OAS3 documents
• Changes the response negotiation to consider the presence of a default response (which gets overridden with the requested status code)
• Changes the media type negotiation to not throw in case there's no content for that (return empty instead)
• Uses the noEmit flag for typechecks in CI so checks time halves.

[XVincentX]

@philsturgeon I've gone ahead, unified the two documents (oas2 and 3) and used the same test assertions; then I wrote the required specific assertions for oas3.

Moreover, while writing these tests I literally lost 2-3 hours in understanding a subtle bug I was not able to track — ultimately it turn out that since the graphite instances created by the loaders for Prism are singleton, when loading a different file even though creating a new Prims instance the new resources get stacked up on top of the last loaded ones — basically creating a whole oas2+oas3 resources array, confusing the whole Prism server. See a33b860 for more details. //cc @chris-miaskowski this made me lose my mind for a while

[chris-miaskowski]

Yo, good job Vincenzo! Prism it starting to look great again :)
I have only few small comments.

[chris-miaskowski]

All good!