revert: @testing-library/react 버전 v15 -> v14 revert

[ssi02014]

Overview

userEvent와 fakeTimer를 함께 사용하면 timeout 이슈 발생
testing-library/user-event#1173 (comment) 비슷한 이슈같은데.. 계속 트래킹 필요

PR Checklist

• All tests pass.
• All type checks pass.
• I have read the Contributing Guide document.
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