Make test params readonly so they can't be accidentally permanently modified

[kainino0x]

This should hopefully categorically prevent bugs like the one fixed in #3096.

Verified this only has the expected changes on the generated JS by comparing the build out directory.

git checkout main
npm run standalone
npx prettier --write out/**/*.js
git add -f out/**/*.js
git checkout readonly-params
npm run standalone
npx prettier --write out/**/*>js
git diff > readonly-params.diff

https://gist.github.com/kainino0x/2851c43552987738ef2e9125389eb092
Only the unittests changed, and of course they pass (they're checked on presubmit).

Issue: None

---

Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

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[lokokung]

Ah... the typing for the tuples is rather nasty lol... The nested arrays is also not the prettiest... Otherwise seems fine. Just out of curiosity, if we pass a super large tuple or quadruply nested array, does it just give a compile error? Or will it do something else?

[kainino0x]

Just out of curiosity, if we pass a super large tuple or quadruply nested array, does it just give a compile error? Or will it do something else?

Nested array should be fine, as it's recursive. If we pass a tuple larger than 7, it will coerce to an array, e.g. [A,B,C,D,E,F,G,H] will become ReadonlyArray<A|B|C|D|E|F|G|H>