chore(next-custom-transforms): Mark fixture outputs as generated (#73918)
This should let GitHub (and Graphite) collapse these files by default in code review.
```
$ git check-attr --all crates/next-custom-transforms/tests/fixture/strip-page-exports/getInitialProps/class/*
crates/next-custom-transforms/tests/fixture/strip-page-exports/getInitialProps/class/output-data.js: linguist-generated: true
crates/next-custom-transforms/tests/fixture/strip-page-exports/getInitialProps/class/output-default.js: linguist-generated: true
```