Remove failing assertions from `is_serialized()` tests.

They have been moved to new test methods, which have been attached as a patch
to the ticket #9930.

See #30284.
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Remove failing `is_textdomain_loaded()` test.

The removed test has been added as a patch to the original ticket, #21319.

Props MikeHansenMe.
See #30284.
Built from https://develop.svn.wordpress.org/trunk@30521


git-svn-id: http://core.svn.wordpress.org/trunk@30510 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove failing test related to `wp_unique_post_slug()`.

The test has been added as a patch to #18962.

See #30284.
Built from https://develop.svn.wordpress.org/trunk@30522


git-svn-id: http://core.svn.wordpress.org/trunk@30511 1a063a9b-81f0-0310-95a4-ce76da25c4cd