Fixup get_currency_name test

This test failed in environments where no default locale
can be determined; make the test deterministic and add
additional test for plurality.

Signed-off-by: James Page <james.page@ubuntu.com>
Merge pull request #85 from prmtl/1.x-maintenance-backport-test-fix

[backport] Fixup get_currency_name test
extracted subdir filter option to a function and add a failing test
addd a fix for filtering unwanted dirs