Update i18n error codes severity

[ernilambar]

PRT team has decided to increase severity for several i18n error codes which are frequently hit in plugin submissions.

This PR:

• Raises the severity for i18n error codes for WordPress.WP.I18n.InterpolatedVariableXXX and WordPress.WP.I18n.NonSingularStringLiteralXXX. Earlier only selective errors codes were assigned severity 7.
• Unit tests have been added.

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Co-authored-by: ernilambar <rabmalin@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
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[swissspidy]

Looking at the diff, it seems these were already at severity 7, no? 🤔

[ernilambar]

Looking at the diff, it seems these were already at severity 7, no? 🤔

Earlier only three were severity 7, WordPress.WP.I18n.InterpolatedVariableDomain, WordPress.WP.I18n.NonSingularStringLiteralContext and WordPress.WP.I18n.NonSingularStringLiteralDomain.

Now other codes will also be updated like WordPress.WP.I18n.InterpolatedVariableXXX. XXX could contain Single, Plural, etc.