Add test to PdfMergeMetadataImporterTest

[paudelritij]

Closes #12000
Added a test method fetchArxivInformationForPdfWithArxivId that assert the expected metadata with actual metadata of a pdf with arXiv ID.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
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[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨