Create a generic unix account test

[chisholm]

The existing unix password and shadow tests are nss database-specific: implementations are only allowed to search the password and shadow databases. But a test shouldn't be tied to a particular implementation. In this new test, all account-related info could be combined; no need to split it up according to implementation details (e.g. nss database). It could also enable searching based on user IDs in addition to names, which is a requested feature.

We need to determine whether there is a standard set of entities that would work across *nix platforms. We may also need scoping rules similar to windows account tests, to avoid hammering external account servers.

See OVAL language tracker #49 and the links to mailing list discussions therein.