main: schema test coverage

[ralfhandl]

Thanks to @jdesrosiers we can now measure the coverage of our schema tests.

Bad news: it is pretty low right now:

• 3.0: Covered: 258 of 531 (48%)
• 3.1: Covered: 220 of 608 (36%)

Shamelessly copied from

• How to measure the test coverage of a schema? hyperjump-io/json-schema#70

[jeremyfiel]

I can help write some of these as I get time.. do you prefer a lot of smaller tests of individual items or combining many different ones into a large OAS file?

i have one available with up to 85+% coverage

[ralfhandl]

For fail test cases I prefer minimalistic files that contain exactly one error.

For pass test cases "maximum" examples covering optional stuff in addition the required minimum are perfectly fine.