[naga] Ensure test functions in glsl snapshots are reachable from the entry point

[andyleiserson]

For #5885, we are planning to change compaction to remove unreferenced functions.

This change ensures that test functions in GLSL snapshots are reachable from the entry point, so that when we start pruning unreachable functions, we do not lose test coverage.

This is the first of three similar changes to tests. The others are for the WGSL snapshot tests and for the non-snapshot naga tests.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.