Move country codes list from IBANInt'lTest to CountryCodesTest.

Commit d9dc659929dac646862417aba7646a78993909bb moved the country codes from
IBAN.java to CountryCodes.java. This commit moves the test data from
IBANInternationalTest.java to CountryCodesTest.java for symmetry.
Adds an "isKnownCountryCode(String)" method to CountryCodes.
Add a meta-test to ensure all known country codes are tested.