Review use of external sniffs

Both VIPCS rulesets currently use the following external sniffs:
```
Generic (5 sniffs)
------------------
Generic.NamingConventions.ConstructorName
Generic.PHP.DisallowShortOpenTag
Generic.PHP.NoSilencedErrors
Generic.PHP.Syntax
Generic.VersionControl.GitMergeConflict

PSR2 (1 sniff)
---------------
PSR2.Files.ClosingTag

Squiz (3 sniffs)
----------------
Squiz.PHP.CommentedOutCode
Squiz.PHP.Eval
Squiz.WhiteSpace.SuperfluousWhitespace

VariableAnalysis (1 sniff)
---------------------------
VariableAnalysis.CodeAnalysis.VariableAnalysis

WordPress (20 sniffs)
---------------------
WordPress.CodeAnalysis.AssignmentInCondition
WordPress.DB.DirectDatabaseQuery
WordPress.DB.PreparedSQL
WordPress.DB.SlowDBQuery
WordPress.DateTime.RestrictedFunctions
WordPress.PHP.DevelopmentFunctions
WordPress.PHP.DiscouragedPHPFunctions
WordPress.PHP.DontExtract
WordPress.PHP.IniSet
WordPress.PHP.StrictComparisons
WordPress.PHP.StrictInArray
WordPress.Security.EscapeOutput
WordPress.Security.NonceVerification
WordPress.Security.PluginMenuSlug
WordPress.Security.ValidatedSanitizedInput
WordPress.WP.AlternativeFunctions
WordPress.WP.CronInterval
WordPress.WP.EnqueuedResources
WordPress.WP.GlobalVariablesOverride
WordPress.WP.PostsPerPage
```

For each of these sniffs, a review should be done to verify the following:
* Is the sniff still relevant ?
* Does the sniff do what it should do ?
* Is this still the best sniff for the job or have other sniffs been published in the mean time which are better, i.e. should the current sniff inclusion be replaced with another sniff ?
* Are there known issues with the sniff, either reported here or upstream ? And if so, can we contribute to fixing these ?
* Are there unreported issues with the sniff based on a quick code review ? Things along the same lines as the VIPCS native sniffs are being reviewed for, like code style independence and correct handling of modern PHP code.

I'm opening this issue as a placeholder/reminder that this review needs to be done. When the time is right to address this, we may need to open individual issues for each sniff to allow for a more targeted discussion of each sniff.

この issue の外部 sniff リストから始め、Generic、PSR2、Squiz、VariableAnalysis、または WordPress rulesets のいずれかから sniff を1つ選択します。2つの VIPCS rulesets および upstream プロジェクトの情報に照らして、その現在の妥当性、動作、代替手段、報告済みまたは観測可能な問題を確認します。各 sniff についてレビューが文書化され、フォローアップ作業が必要な場合に個別の対象を絞った issue が起票されていれば完了です。

索引モデルが issue の本文から書いたものです。