Tests: Remove uploads added by REST API media tests

[jnylen0]

Before this change, running the WP_REST_Attachments_Controller tests multiple times in a row fails because these tests do not clean up their uploaded files. I do not see this error when running the full suite and I'm not sure why, but these files should still be cleaned up.

Here is the error that happens on the 2nd run of these tests:

$ phpunit --filter WP_Test_REST_Attachments_Controller
Installing...
Running as single site... To run multisite, use -c tests/phpunit/multisite.xml
Not running ajax tests. To execute these, use --group ajax.
Not running ms-files tests. To execute these, use --group ms-files.
Not running external-http tests. To execute these, use --group external-http.

PHPUnit 4.8.22 by Sebastian Bergmann and contributors.

.....................................F........................

Time: 4.9 seconds, Memory: 172.25Mb

There was 1 failure:

1) WP_Test_REST_Attachments_Controller::test_create_item_default_filename_title
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'codeispoetry'
+'codeispoetry-1'

[johnbillion]

This happens in other tests too. See #38264.

[rachelbaker]

In 39041:

REST API: Delete temporary files during teardown of WP_Test_REST_Attachments_Controller.

Props jnylen0.
Fixes #38574.