Moves defintion of ajax request in `WP_Ajax_UnitTestCase` to `setUp()` method to account for hooks being reset as part of `tearDown()`.
Props matthias.thiel for report.
Fixes#41561.
git-svn-id: https://develop.svn.wordpress.org/trunk@41970 602fd350-edb4-49c9-b593-d223f7449a82