Tests: Avoid physical image uploads in Tests_Admin_ExportWp

[SainathPoojary]

Optimizes fixture setup in Tests_Admin_ExportWp by avoiding physical image uploads and thumbnail generation during sample data preparation.

As noted in the ticket description for #66074, Tests_Admin_ExportWp is driven by fixture and I/O cost. Previously, wpSetUpBeforeClass() called create_upload_object(), which physically copied four JPEGs to the uploads folder and generated intermediate image sizes for each of the 11 tests running in separate PHP processes.

export_wp() tests only assert that post and attachment database records appear correctly in the exported XML, they do not inspect physical files or image dimensions. Replacing create_upload_object() with $factory->attachment->create() creates the necessary database records and thumbnail relationships directly, eliminating disk writes and GD/Imagick sub-size generation.

Performance

Local measurements across three runs (each running 11 tests across separate processes):

Run	Trunk Test Time	Trunk Wall Time	Fix Test Time	Fix Wall Time
Run 1	10.63s	34.17s	7.96s	27.24s
Run 2	8.01s	26.46s	8.36s	26.65s
Run 3	9.01s	29.13s	8.53s	29.20s
Average	9.22s	29.92s	8.28s	27.70s

• Average JUnit runtime: ~10.1% reduction (from 9.22s to 8.28s).
• Average PHPUnit runtime: ~7.4% reduction (from 29.92s to 27.70s).
• Setup time per process: Dropped from ~0.72s to ~0.60s.

While the local time gain is modest and subject to process variance, eliminating disk I/O and thumbnail generation removes unnecessary filesystem churn across all separate test processes.

Validation

• All 11 tests and 35 assertions pass on both single-site and multisite.
• Verified attachment parent relationships and featured image IDs (set_post_thumbnail) are intact.
• PHPCS passes with 0 errors and 0 warnings.

Trac ticket: https://core.trac.wordpress.org/ticket/66074

Use of AI Tools

AI assistance: Yes
Tool(s): Github Copilot
Used for: Edge-case checks, and assistance with the PR description. Implementation and testing were made and reviewed by me.

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[lancewillett]

The simplification looks reasonable.

Before landing, please investigate the PHPUnit failures.

I reproduced that failure locally when running the export tests, the IncludesMedia upload fixtures, and the REST filename tests together.

Upload cleanup and the $ignore_files snapshot look worth investigating. A comparison didn’t guarantee clean uploads, so I haven’t established whether this PR introduces or exposes the problem.

Please resolve the failure and get CI green before we approve and commit this.