Bump ajv from 6.10.0 to 6.12.6 in /test/fixture/v4.0.x

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.10.0 to 6.12.6.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](ajv-validator/ajv@v6.10.0...v6.12.6)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
...

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