Install test dependencies for benchmarks

[radarhere]

#10205 didn't consider that the Benchmarks CI job was relying on the Install step, meaning that olefile is not available - https://github.com/python-pillow/Pillow/actions/runs/38696429958/job/116294282528.

The reason the error is occurring now, rather than in the PR, is the addition of #10197. See #10213 for making olefile optional again.

[codspeed]

Merging this PR will degrade performance by 11.37%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 4 (👁 3) regressed benchmarks
✅ 636 untouched benchmarks
⏩ 338 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_get_flattened_data[1237x811-L]	238.3 ms	267.4 ms	-10.88%
👁	test_draw_lines_blend[1237x811-0]	1 ms	1.2 ms	-11.58%
👁	test_draw_lines_blend[1237x811-127]	1 ms	1.2 ms	-11.46%
👁	test_draw_lines_blend[1237x811-255]	1 ms	1.2 ms	-11.56%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing radarhere:benchmark (50d6ed2) with main (bc7e6f5)²}

Footnotes

1. 338 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (964d7bc) during the generation of this report, so bc7e6f5 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩