fix(tests): remove deprecated argument from pytest_report_header

[gg-jonathangriffe]

This MR fixes pytest_report_header. The startdir argument was deprecated and removed in the new pytest release.

[codecov-commenter]

Codecov Report

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

Project coverage is 91.76%. Comparing base (937983e) to head (53ed480).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #859 +/- ## ======================================= Coverage 91.76% 91.76% ======================================= Files 170 170 Lines 7058 7058 ======================================= Hits 6477 6477 Misses 581 581 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/859/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/859/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.76% <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=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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