Fix for failing PHPUnit tests cases

[dextorlobo]

Some PHPUnit Tests are failing in wordpress-develop repo due to dummy images are not returning expected results.

Steps to reproduce:

• Raise a new PR in wordpress-develop repo against trunk
• Automatic tests start run
• Some tests will get fail

Screenshot of issue:

Fixes:
I have replaced the dummy images which were causing the issue with other dummy.

Trac ticket: https://core.trac.wordpress.org/ticket/60865

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[speedyprem]

Thanks for the @dextorlobo. Code looks good 👍

[SergeyBiryukov]

Thanks for the PR! Merged a slightly modified version in r57903.