The test
WC_Tests_API_Functions::test_wc_rest_upload_image_from_url_should_download_image_and_return_array()
was failing on Travis with the error below due images being kept on
accros different tests. To fix this problem, this commit makes sure that
every test that creates an image, removes it after the assertions.
```
1) WC_Tests_API_Functions::test_wc_rest_upload_image_from_url_should_download_image_and_return_array
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
Array (
- 'file' => '/tmp/wordpress/wp-content/uploads/2020/01/Dr1Bczxq4q.png'
- 'url' => 'http://example.org/wp-content/uploads/2020/01/Dr1Bczxq4q.png'
+ 'file' => '/tmp/wordpress/wp-content/uploads/2020/01/Dr1Bczxq4q-3.png'
+ 'url' => 'http://example.org/wp-content/uploads/2020/01/Dr1Bczxq4q-3.png'
/home/travis/build/woocommerce/woocommerce-rest-api/unit-tests/Tests/Version3/functions.php:113
```
(https://travis-ci.org/woocommerce/woocommerce-rest-api/jobs/634921426#L719)
Rodrigo Primo
Christopher Allford
Mike Jolley
