fix: disallow `#[test]` on associated functions #15134

Job	Run time
build-nargo	2m 28s
build-noirc-abi	1m 42s
build-acvm-js	1m 22s
build-noir-wasm	3m 46s
ACVM JS (Browser)	1m 18s
ACVM JS (Node.js)	42s
noirc_abi	1m 39s
Noir JS	1m 14s
Example scripts	16s
noir_codegen	1m 2s
Check external repo - ${{ matrix.project.repo }}	0s
Integration Tests (Node)	2m 19s
noir_wasm	1m 32s
Integration Tests (Browser)	1m 55s
End	1s
	21m 16s

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