adding tests for Bank::BIC and making valid_format? return a boolean

valid_format? uses a regular expression for comparing the BIC code, which currently returns an integer or nil (if it doesn't match).

The result of this is that a BIC will be "nil" for valid? if the format doesn't match. This change makes valid_format? return a boolean value so that it will evaluate as expected downstream (and also agrees with the naming convention of the method ending in a question mark).