PHPUnit 9.6 へのバージョンアップ

[nanasess]

Fixes #1020

• [x] Warning の解消
• [x] カバレッジの確認

[nanasess]

テストのエラーは #1018 で解消予定

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 51.66%. Comparing base (6c22132) to head (efe6ab3). Report is 14 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1022 +/- ## ========================================== - Coverage 56.23% 51.66% -4.57% ========================================== Files 75 80 +5 Lines 9046 10155 +1109 ========================================== + Hits 5087 5247 +160 - Misses 3959 4908 +949 ``` | [Flag](https://app.codecov.io/gh/EC-CUBE/ec-cube2/pull/1022/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EC-CUBE) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/EC-CUBE/ec-cube2/pull/1022/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EC-CUBE) | `51.66% <ø> (-4.57%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EC-CUBE#carryforward-flags-in-the-pull-request-comment) to find out more.

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[nanasess]

php-cs-fixer を最新にすると影響が大きいため、別 PR で対応します