Bump test/fixtures/prism from 1c8d08b to b6aa0f2

Bumps [test/fixtures/prism](https://github.com/ruby/prism) from `1c8d08b` to `b6aa0f2`.
- [Release notes](https://github.com/ruby/prism/releases)
- [Commits](ruby/prism@1c8d08b...b6aa0f2)

---
updated-dependencies:
- dependency-name: test/fixtures/prism
  dependency-type: direct:production
...

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