[php-src] PR #22987: ext/gmp: applied fixers to improve test robustness

Pull Request: https://github.com/php/php-src/pull/22987 Author: NickSdot Hey! As requested in https://github.com/php/php-src/pull/22799#issuecomment-5124595234 I carry over commits into separate PRs for extensions. This was cherry picket from https://github.com/php/php-src/pull/22799 (where the deterministic proof is). I manually reviewed the diff, this PR is not affected by the ongoing style discussion, there are zero (0) style-only changes. Hence, this is safe to merge independently. The more individual extension PRs are merged, the smaller the diff in the main PR, the easier to review. Merging this one will decrease diff size in the main PR by ~1,000 lines (~2.5%), and will decrease its affected files by 65 files (~2.3%).