This commits changes WC_Unit_Test_Case parent class to WP_HTTP_TestCase (which extends WP_UnitTestCase). This way all WC core test classes can benefit from the functionality provided by WP_HTTP_TestCase if needed. This is necessary because otherwise test classes can use the functionality provided by WC_Unit_Test_Case or WP_HTTP_TestCase. This change should not affect test classes that don't explicitly call one of the WP_HTTP_TestCase features.
This commit changes the WC_Tests_Product_CSV_Importer class to use artificial HTTP responses instead of performing real HTTP requests to a external server to get the images of the imported products. On my local machine, this change reduced the test execution time from 3257ms to 831ms.
As a consequence of this change, this commit also fixes WC_Tests_Product_CSV_Importer::test_import() in all Travis builds. This test has been failing for a few weeks on Travis but not on our local environments. After some debugging, I found out that the test was failing on Travis because of the following error when `wp_safe_remote_get()` was called to get images for the imported products:
```
Error getting remote image http://demo.woothemes.com/woocommerce/wp-content/uploads/sites/56/2013/06/T_1_front.jpg. Error: cURL error 35: gnutls_handshake() failed: Handshake failed.
```
Apparently, the PHP binary that is used by Travis is unable to handle the TLS handshake (see https://github.com/travis-ci/travis-ci/issues/6339) and the test probably started failing when demo.woothemes.com (which is used to download the images for the imported products) switched from HTTP to HTTPS.
This commits adds a call to `parent::setUp()` inside WC_Tests_Product_CSV_Importer::setUp(). This is necessary to make sure transactions are used on database calls and thus tests don't have to worry about cleaning inserted data.
I found this issue while debugging https://github.com/woocommerce/woocommerce-product-tables-feature-plugin/issues/81. Turns out this test was failing because WC_Tests_Product_CSV_Importer::test_import() was leaving one post in the database due to a bug in the custom product tables plugin.
Rodrigo Primo
Gerhard Potgieter
Claudio Sanches
claudiulodro
Mike Jolley
