3.2 schema tests: keyword coverage

[ralfhandl]

3.2 port of

• 3.1 schema tests: keyword coverage #4839

Raise keyword (statement) coverage of our OAS schemas to 100% by adding "pass" test cases:

• Link Object with requestBody

• Schema Object with (deprecated) example keyword

• Specification Extension

• technical test: Schema Object via schema.yaml instead of schema-base.yaml

• no schema changes are needed for this pull request

[ralfhandl]

Merged with one approval because it only adds test cases