Tests: Delete leftover image sub-sizes after WP_Customize_Manager tests.

After running `phpunit --filter test_import_theme_starter_content`, the following files were generated and left in place in the version-controlled `DIR_TESTDATA` directory, resulting in a dirty working copy of the project:
{{{
tests/phpunit/data/images/canola-150x150.jpg
tests/phpunit/data/images/canola-300x225.jpg
}}}

In the test, `canola.jpg` is mocked as an **existing attachment**. When the test invokes `WP_Customize_Manager::import_theme_starter_content()`, existing attachments are passed directly to `wp_generate_attachment_metadata()`, which does not make a copy of the file, and `wp_create_image_subsizes()` creates the above referenced files in the original image's location.

By contrast, `waffles.jpg`, also used in the test, is **not** set as an existing attachment, and in  `import_theme_starter_content()` is passed to `media_handle_sideload()`, which makes a **copy** of the file in the `wp-content/uploads/` directory, and then calls `wp_create_image_subsizes()` against the new file. The resulting sub-sizes are generated in that location, and are cleaned up during `tear_down()`.

The test's `tear_down()` uses `remove_added_uploads()`, which only clears items created in `wp-content/uploads/` (i.e. `waffles*.jpg`), but not the files in the `DIR_TESTDATA` directory (`canola-*.jpg`).

This commit addresses the issue by creating a copy of the file in uploads. This better matches the intention of the test, as existing attachments will most likely be located in uploads and not in an arbitrary path like the test data directory.

Follow-up to [39276], [39346], [39411], [40142], [54424], [54425].

Props ironprogrammer, audrasjb, boniu91, dariak, SergeyBiryukov.
Fixes #56807.
Built from https://develop.svn.wordpress.org/trunk@54519


git-svn-id: http://core.svn.wordpress.org/trunk@54074 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-RC1-54518';
+$wp_version = '6.1-RC1-54519';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.