Tests for image thumbnail processing should not rely on a static filename

[jeremyfelt]

As of [33181], builds in Travis are failing during multisite tests due to a mismatch in filename between test-image.jpg and test-image1.jpg. It appears this may be the case because of how fast Travis runs the tests as I am unable to reproduce locally.

In test_insert_image_no_thumb(), we're currently checking that the returned filename is test-image.jpg, when it could be changed by wp_upload_bits() if a conflict exists. The new site icon tests also make use of test-image.jpg, which is likely where wires started crossing.

[jeremyfelt]

In 33188:

Tests: Check downsized images against dynamic rather than static filename when original is expected.

See #32971.

[jeremyfelt]

Everything is ​passing after [33188].

In the future we may want to explore using rand_str() or something for all filenames created in the upload directory.