test: unflakify test cases

[Kha]

With the recent unification of server and cmdline processing, IO.Process tests that previously broke the server because they directly wrote to stdout are now flaky on the cmdline because elaboration and reporting are happening in separate threads. By removing direct writes to stdout, the race condition is removed and the file can actually be edited in the language server as well again.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 574066b30b32ed9742042706b59ea0c12d0541d8 --onto 647a5e94925791f6a16b629b6c16ffad60a7f478. (2024-08-07 08:36:18)