Improve test coverage of the I18n class

[36degrees]

Because Intl.PluralRules exists in the test environment, we didn't have any coverage for these lines of code:

govuk-frontend/src/govuk/i18n.mjs

Lines 168 to 182 in 7fac761

	// Currently our custom code can only handle positive integers, so let's
	// make sure our number is one of those.
	count = Math.abs(Math.floor(count))
	// Look through the plural rules map to find which `pluralRule` is
	// appropriate for our current `locale`.
	for (var pluralRule in this.pluralRulesMap) {
	if (Object.prototype.hasOwnProperty.call(this.pluralRulesMap, pluralRule)) {
	var languages = this.pluralRulesMap[pluralRule]
	if (languages.indexOf(locale) > -1 || languages.indexOf(localeShort) > -1) {
	keySuffix = this.pluralRules[pluralRule](count)
	break
	}
	}
	}

This refactors the I18n class to split out and test the 'fallback' logic that:

• determines which 'rules' to use to determine the plural rule (eurgh, too many things we're calling rules) based on locale
• uses the rules to determine the plural rule…

According to Jest, we now only have 2 lines of code that are not tested:

Line 158 (we could mock out hasIntlPluralRulesSupport to return false if we think it's needed?)

govuk-frontend/src/govuk/i18n.mjs

Line 158 in 7f73c53

return this.selectPluralRuleFromFallback(count)

Line 302 (As far as I can tell this code is unreachable):

govuk-frontend/src/govuk/i18n.mjs

Lines 294 to 303 in 7f73c53

	russian: function (n) {
	var lastTwo = n % 100
	var last = lastTwo % 10
	if (last === 1 && lastTwo !== 11) { return 'one' }
	if (last >= 2 && last <= 4 && !(lastTwo >= 12 && lastTwo <= 14)) { return 'few' }
	if (last === 0 || (last >= 5 && last <= 9) || (lastTwo >= 11 && lastTwo <= 14)) { return 'many' }
	// Note: The 'other' suffix is only used by decimal numbers in Russian.
	// We don't anticipate it being used, but it's here for consistency.
	return 'other'
	},

I've also moved the I18n.pluralRulesMap and I18n.pluralRules from the prototype as they feel more like 'class constants' than properties that need to exist on every object.

Suggest reviewing commit by commit as I've moved some stuff around.

[colinrotherham]

Work of art 🙌