Upgrade/Install: Properly exclude the `admin/includesSchema.php` tests file...

Upgrade/Install: Properly exclude the `admin/includesSchema.php` tests file for the `WordPress.DB.PreparedSQL.NotPrepared` sniff.

The way this was implemented in [43630] caused the file to be excluded for all sniffs, since PHPCS does not support file-wide ignores of a specific sniff at this point.

Props jrf.
See #44896.

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git-svn-id: https://core.svn.wordpress.org/trunk@43470 1a063a9b-81f0-0310-95a4-ce76da25c4cd