front: set timezone for test

[sim51]

Tests are failing locally because I'm not in the UTC timezone. So tests are plateform dependent, which is bad. This commit sets the timezone when runnning vitest.

[codecov-commenter]

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Codecov Report

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

Project coverage is 28.34%. Comparing base (b5e1c9d) to head (a15ea86). Report is 1 commits behind head on dev.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #7961 +/- ## ============================================ - Coverage 28.35% 28.34% -0.01% Complexity 2075 2075 ============================================ Files 1270 1270 Lines 155669 155672 +3 Branches 3076 3076 ============================================ - Hits 44137 44129 -8 - Misses 109699 109710 +11 Partials 1833 1833 ``` | [Flag](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [core](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `75.02% <ø> (ø)` | | | [editoast](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `71.25% <ø> (-0.03%)` | :arrow_down: | | [front](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `9.95% <ø> (-0.01%)` | :arrow_down: | | [gateway](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `2.34% <ø> (ø)` | | | [railjson_generator](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `87.49% <ø> (ø)` | | | [tests](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `72.93% <ø> (ø)` | | 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=None#carryforward-flags-in-the-pull-request-comment) to find out more.

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[sim51]

Tested on linux & windows with WSL (thanks Sarah)