Bump test/fixtures/prism from aa42f45 to b6a97a4

Bumps [test/fixtures/prism](https://github.com/ruby/prism) from `aa42f45` to `b6a97a4`.
- [Release notes](https://github.com/ruby/prism/releases)
- [Commits](ruby/prism@aa42f45...b6a97a4)

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

Signed-off-by: dependabot[bot] <[email protected]>