Acceptance tests now end in .atest.js.

They're different enough from normal tap tests that I don't think they
should have the .test.js extension, but I would like to maintain that
naming style, so .atest.js seemed reasonable.

Also imported the acceptance test function as 'acceptanceTest' instead
of 'test' in login.atest.js, to make sure they don't get confused with
standard tap tests.