Build/Test Tools: Add an extra `WP_Error` assertion when testing a valid user...

Build/Test Tools: Add an extra `WP_Error` assertion when testing a valid user activation key. This provides a better failure message if the assertion does fail.

See #38716

Built from https://develop.svn.wordpress.org/trunk@39364


git-svn-id: https://core.svn.wordpress.org/trunk@39304 1a063a9b-81f0-0310-95a4-ce76da25c4cd