Tests: Fix tests failing due to `assertContains()` using strict checking.

Since PHPUnit 8.0.2, the `assertContains()` method, when checking whether a value exists in an array, will do a strict type comparison of the values.

This caused a couple of tests to fail. Using the correct data type in the test fixes that.

References:
* https://github.com/sebastianbergmann/phpunit/blob/8.0.6/ChangeLog-8.0.md#802---2019-02-07
* https://github.com/sebastianbergmann/phpunit/issues/3511
* 6205f33595

Follow-up to [51559-51570].

Props jrf.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51571


git-svn-id: http://core.svn.wordpress.org/trunk@51182 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51570';
+$wp_version = '5.9-alpha-51571';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.