(#12412) Refactor puppet_resource('user') tests

Previously the Linux-specific logic for validating
puppet_resource('user') was contained within each 'user' test case. In
order to support executing resource acceptance tests on non-Linux
agents, I could have added if-then-else statements to each test case,
though that would lead to logic-sprawl.

This commit refactors the existing set of 'user' acceptance tests to
use native commands to validate that the tests work on any supported
agent platform. Specifically, that puppet_resource('user') can create,
modify, delete, query, and query_all, and that it doesn't try to
delete a non-existent user, or add a user that already exists.

The goal of these tests is to validate that puppet_resource "works"
with user resources, and can therefore be trusted to validate the
results of subsequence acceptance tests.

The commit also preserves tests that exercise unique behaviors,
e.g. should_create_with_gid.