FileSystemWritesDisallow: Use the WPCS abstract function call sniff

[jrfnl]

There were three sniffs which still extended the upstream Generic_Sniffs_PHP_ForbiddenFunctionsSniff class.
Extending the WordPress_AbstractFunctionRestrictionsSniff instead has the following advantages:

• Functions can be provided in groups and the exclude property to selectively exclude groups becomes available.
• The sniffs no longer have a public $forbiddenFunctions property which could be overruled via the ruleset.
• Once callback checks have been added to the WordPress_AbstractFunctionRestrictionsSniff, the sniffs will benefit from the additional detection. (see Restricted functions used in callbacks #611)
• If needed at some point, the functions within the WordPress_Sniff class are available for use.

This PR addresses the above for the VIP.FileSystemWritesDisallow sniff.

This PR contains no significant functional changes. The sniff effectively still does the same.

Notes:

• The errorcode changes to the groupname instead of being fixed at FileWriteDetected.
• Split up the functions array into logical groups and alphabetized the lists.
• Removed some duplicates from the functions array.
• Added additional unit tests to make sure all functions being sniffed for are unit tested

Related to #614