Split the API test suites into Item and List classes.

For organizational purposes, and to make it easy to select a particular
set of tests, the API test classes have been split into two. There are
now resource list tests in each test file (all named ResourceListTests),
and item tests (ResourceItemTests). By keeping the same name, there's no
guesswork. It's easy to just run a set of tests in a resource's test
file.

While in here, I removed some usage of test_scmtools from the new
classes, which were not really needed.