Tests: Always include the error message in `assertNotWPError()` and `assertNotIXRError()`.

Previously, in case of failure, `WP_UnitTestCase_Base::assertNotWPError()` displayed the actual error message from the passed `WP_Error` object, but only if the `$message` parameter was empty.

This made the assertion less helpful, as the actual error message was lost in case there was a non-empty `$message` parameter passed to the method, as per the [https://make.wordpress.org/core/handbook/testing/automated-testing/writing-phpunit-tests/#using-assertions Writing PHP Tests] guidelines:
> All PHPUnit assertions, as well as all WordPress custom assertions, allow for a `$message` parameter to be passed. This message will be displayed when the assertion fails and can help immensely when debugging a test. This parameter should always be used if more than one assertion is used in a test method.

This commit ensures that the actual error message is always displayed, in addition to the passed `$message` parameter.

The same applies to `WP_UnitTestCase_Base::assertNotIXRError()`.

Follow-up to [34638], [40417].

Props jrf, SergeyBiryukov.
See #55652.
Built from https://develop.svn.wordpress.org/trunk@53536


git-svn-id: http://core.svn.wordpress.org/trunk@53125 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53535';
+$wp_version = '6.1-alpha-53536';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.