3.1 schema tests: keyword coverage

[ralfhandl]

Raise keyword/line 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

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request

[ralfhandl]

Merged with one approval because it only adds test cases