Add test for getRequiredDirectories

See https://trello.com/c/cvJ4VV9x/45-getrequireddirectories-testing.  This updates the data tests to check if the required directory count is too small; it also has the side effect of testing file name mapping support.

Several readers also required fixes to make the test pass, primarily due to issues with file mapping.  No special testing is required, other than to verify that builds remain green.