Extend L10n testing to message IDs in “lib/rules.json” too

[tripu]

Right now, test/l10n.js matches two types of strings:

• Info/warning/error messages being emitted in lib/rules/*, eg sr.error(self, 'messageID'); (heuristically; it cannot find all occurrences).
• Message IDs in lib/l10n-en_GB.js (this is exact; all are found).

These tests should also take into account self.rule and make sure it exists in lib/rules.json, doing some kind of 3-way check.

This would have prevented the errors that #425 fixed.

[tripu]

Also, “at some point, we should make sure the checks described in rules.json have an actual rule associated with it.”